Why is prothrombin time test not used for heparin

Why is prothrombin time test not used for heparin

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Prothrombin time test is used for the monitoring of warfarin but not heparin. Why is this used for warfarin monitoring and why is it not used for heparin monitoring?

Thats because the mechanism of action is different between warfarin and heparin. Warfarin inhibits the formation of prothrombin (also known as factor II) by inhibiting the metabolization of vitamin K (for extensive details have a look at the Wikipedia article about Warfarin).

Heparin inhibits the blood coagulation by binding to antithrombin III which functions as a inactivator of active clotting factors as factors V, IX, X, XI and XII. It also binds calcium which is important for the blood coagulation. For more details see the Wikipedia under Heparin.

Therefore Heparin interferes with the formation of Thrombin and the test does not work. A nice image of the coagulation cascade can be found here, which also shows the points where Heparin interferes. The test can be done with Batroxobin, see here.

The answer and reasoning by Chris are incorrect. Heparin DOES effect both the PT time and APTT time because it affects factors in the final common pathway. Heparin activates antithrombin 3 which inactivates activated factor x (factor xa) and thrombin (AKA activated factor ii). These factors are involved in the final common pathway: activated factor x converting prothrombin to thrombin which converts fibrinogen to fibrin. Both the APTT and PT rely on the final common pathway which follows on from either the intrinsic pathway (in APTT) or extrinsic (in PT).

HOWEVER, heparin does have a much greater effect on APTT and so this is why this is used. Indeed at therapeutic concentrations PT is within normal range due to a minimal increase. However, in a heparin overdose PT will become significantly high.

I am not sure of the reason for heparins greater effect on APTT but it may be something to do with heparins mechanism of action - in addition to inactivating factor xa and thrombin, antithrombin 3 (which heparin activates) inactivates other factors (xia and ixa apparently) which are part of the intrinsic pathway (which is measured in APTT) but not the extrinsic pathway (measured in PT). I think the fact it affects multiple factors in the APTT test causes a synergistic affect so affects it more compared to the PT where it only affects two factors (xa and thrombin) so there isn't the synergistic effect.

I thought of this explanation using an explanation I found of why low molecular weight heparin (LMWH) does not have as much of an effect on APTT as unfractionated heparin (UFH). LMWH's, unlike UFHs, activation of antithrombin 3 only really causes inactivation of factor xa so the synergistic affect of effecting multiple clotting factors involved in APTT does not happen (like it does with unfractionated heparin). However, for whatever reason in real patients LMWH is as effective at anticoagulation as UFH.

Source of last paragraph :

Standardization of coagulation tests

A lot of attempts have been made to standardise both activated partial thromboplastin time (APTT) and prothrombin time (PT). Only the standardization of PT has been successfully implemented while the standardization of APTT is still underway. The PT test is a common method for monitoring oral anticoagulant therapy. Owing to the variable response of the thromboplastins and the different ways of reporting, PT results obtained from patients treated with oral anticoagulants have not been interchangeable between laboratories. In 1977, the World Health Organization (WHO) designed a batch of human brain thromboplastin as the first international reference preparation (IRP) for thromboplastin and a calibration system was proposed in 1982, based on the assumption that a linear relationship exists between the logarithm of the PT obtained with the IRP and test thromboplastins. This calibration model is used to standardize the reporting of the PT by converting the PT ratio observed with the local thromboplastin into an International Normalized Ratio (INR). The INR system is being adopted by an increasing number of hospitals in many countries. With the increasing use of the INR system, a number of problems have been identified with the INR system. The most serious one is that the ISI of a thromboplastin depends on the coagulometer used. Besides, a number of investigators have noted that the ISI value provided by the manufacturer for each new batch of thromboplastin reagent may be incorrect and the use of inappropriate control plasma can lead to erroneous INR calculations. Four solutions have been proposed to solve the problems of the INR system as follows: (a) the local system calibration with lyophilized plasma calibrants with assigned manual PT determined in terms of the relevant IRP for thromboplastin (b) the use of a mean normal prothrombin time (MNPT) obtained with the coagulometer to derive the prothrombin ratio: (c) PT standardization by means of the procedure using plasma calibrants: and (d) selection of sensitive thromboplastin with low ISI values. The INR system has been adopted in Thailand since 1984. There are 3 steps in the implementation as follows: (a) preparation of National Reference Thromboplastin (b) selection of high sensitive thromboplastin and (c) optimal therapeutic range for Thai patients. The anticoagulant effect of heparin is usually monitored by the APTT, a test that is sensitive to the inhibitory effects of heparin on thrombin, factor Xa. and factor IXa. However, the type of clot detection system, the contact activator and the phospholipid composition of the reagent affect the APTT response. In 1995. ISTH/ICSH proposed a calibration model for APTT standardization. As the problem showed a great similarity to PT standardization, the same model of calibration was applied but no international reference preparation for the APTT is yet available. In 1998. van den Besselaar et al proposed a lyophilized APTT reagent comprising synthetic phospholipids and colloidal silica as a candidate IRP for the APTT.

Which factors can raise the prothrombin time in a blood test?

There are some health circumstances or drugs than can raise the prothrombin time in a blood test:

  • Alcohol
  • Vitamin K deficiency
  • Diarrhea
  • Vomiting
  • Drugs
    • Antacids
      • Cimetidine
      • Amiodarone
      • Metronidazole
      • Tetracycline
      • Asparaginase
      • Acenocoumarol
      • Dabigatran
      • Dicoumarol
      • Heparin
      • Warfarin
      • Colestyramine
      • Clofibrate
      • Allopurinol

      Why Would I Need One?

      Your doctor might order this test to check for a bleeding disorder, such as hemophilia or von Willebrand disease. Symptoms of bleeding disorders include:

      • Bleeding or bruising easily
      • Blood clots that form when they shouldn’t
      • Blood in your poop or urine
      • Gums that bleed easily
      • Heavy menstrual periods in women
      • Nosebleeds
      • Swelling or pain in your joints

      You’d also need this test if you get heparin therapy -- that’s a medicine used to prevent or treat blood clots after you’ve had an issue like a heart attack or stroke. The PTT test helps make sure you get the right dose. You want to prevent dangerous clots, but still let your blood clot when you need it to.

      You may also get the test to:

      • Check that your blood clots normally before you go in for surgery
      • Look for a problem with your immune system (some immune system conditions make clots more likely to form -- in women, that can also lead to miscarriages)
      • See how well your liver’s working, since it makes the clotting factors


      Are baseline prothrombin time/partial thromboplastin time values necessary before instituting anticoagulation?

      Study objectives: To determine whether baseline prothrombin time (PT) or partial thromboplastin time (PTT) values provide information that is useful to the clinician before initiating anticoagulation and whether emergency physicians elicit historical information about bleeding disorders before beginning anticoagulant therapy.

      Design: A three-year retrospective review of the records of 199 patients admitted through the ED with an ICD-9-CM diagnosis of deep-vein thrombosis or pulmonary embolus using a predesigned study sheet that included historical questions, baseline PT and PTT values, treatment given, timing of treatment, and underlying medical problems.

      Setting: University-affiliated tertiary-care hospital.

      Measurements and main results: Deep-vein thrombosis was the primary diagnosis in 75% of patients. Pertinent historical items were not documented in 92% to 100% of patients. Baseline PT and PTT values were obtained in 94% of patients. An elevated baseline PT was found in 26 patients, all of whom were taking warfarin. An elevated baseline PTT was found in 21 patients. These results were attributed to laboratory error (one), warfarin use (nine), heparin therapy before baseline tests (five), anticardiolipin antibodies (five, one of whom was on warfarin therapy), and unknown causes (three). Heparin therapy was not altered for any patient.

      Conclusion: Emergency physicians rarely document pertinent questions about bleeding disorders before initiating anticoagulation therapy. Baseline PT and PTT values are almost routinely obtained despite the fact that they do not alter therapy or serve as sensitive or specific screening tests. Routine baseline PT and PTT values are rarely needed before initiating anticoagulation. Eliminating such routine testing would result in significant cost savings.

      What Abnormal Results Mean

      If you are not taking blood thinning medicines, such as warfarin, an INR result above 1.1 means your blood is clotting more slowly than normal. This may be due to:

      • Bleeding disorders, a group of conditions in which there is a problem with the body's blood clotting process.
      • Disorder in which the proteins that control blood clotting become over active (

      disseminated intravascular coagulation

      If you are taking warfarin to prevent clots, your provider will most likely choose to keep your INR between 2.0 and 3.0:

      • Depending on why you are taking the blood thinner, the desired level may be different.
      • Even when your INR stays between 2.0 and 3.0, you are more likely to have bleeding problems.
      • INR results higher than 3.0 may put you at even higher risk for bleeding.
      • INR results lower than 2.0 may put you at risk for developing a blood clot.

      A PT result that is too high or too low in someone who is taking warfarin (Coumadin) may be due to:

      • The wrong dose of medicine
      • Drinking alcohol
      • Taking certain over-the-counter (OTC) medicines, vitamins, supplements, cold medicines, antibiotics, or other medicines
      • Eating food that changes the way the blood-thinning medicine works in your body

      Your provider will teach you about taking

      Test Details

      How is a PT test performed?

      A PT test is a simple procedure. Your healthcare provider will take a blood sample from your arm (inside the elbow) or your hand. The lab will then time how quickly your blood clots, in seconds.

      How do I prepare for a PT test?

      Very little preparation is necessary for a PT test. However, if you are on blood thinners, you will be watched after the test to check for excessive bleeding.

      A PT test is a very safe procedure. However, there is a small risk of infection, a hematoma, or feeling faint after any blood draw.

      Your healthcare provider may also ask you to stop taking medicine that can affect the test results prior to your appointment. These include aspirin, heparin, antihistamines, and vitamin C.


      • Laboratories should perform assays on factor-deficient plasmas to ensure that each new batch has <1 iu/dl of the deficient factor and normal levels of VWF in FVIII-deficient plasmas being used for Nijmegen-modified Bethesda inhibitor assays.
      • In-house patient plasma should not be used as a source of reagent material.
      • Laboratories should locally determine the effect of lupus anticoagulant on the PT and APTT reagents used in OSCA. Results should be scrutinised for parallelism which may also be lost in patients with other immediate-acting inhibitors.
      • If non-severe haemophilia A is a possibility a two-stage chromogenic assay (CA) for FVIII should be performed in addition to an OSCA, to ensure detection of all cases and to correctly assess the severity.
      • Factor assays (OSCA or CA) should be avoided in patients taking direct FXa or direct thrombin inhibitors. Charcoal-based reversal agents may be of use.
      • Haemophilia centres should ensure that appropriate laboratory assays are available for FVIII and FIX products in local clinical use.

      High prothrombin time and INR results explanation

      In case of Pregnancy PT with aPTT tests requested by gynecologist and the results may show an elevation in Protime only, usually aPTT is within normal.
      In case of Bleeding and clotting disorders as in cardiovascular diseases and valves replacing patients the PT and aPTT are needed with other coagulation function tests, usually it should be normal of ordinary patients and raise gradually as the case be worse.
      In case of Surgery PT,aPTT tests is requested.

      PT test is the inverse relationship between the bleeding time and the hematocrit is particularly Prothrombin Time and Activated Partial Thromboplastin Time.
      PTT (aPTT): (activated) partial thromboplastin time, tells you if patient on heparin is at a therapeutic level.
      INR is made to unify PT time results between different Laboratories by internationalized normal ranges (INR) and calculated as the ratio of a patient’s prothrombin time to a normal (control) sample, raised to the power of the ISI value for the control sample used.
      Thrombocytopenia is a Relationship between platelet count and bleeding time and means low thrombocytes count (Platelets).

      An example of PT test results is:
      Prothrombin Time:
      Control – 12 seconds
      Patient – 18 seconds
      Percent Activity – 34%
      International Sensitivity index – 1.1
      International normalize ratio – 1.6
      Prothrombin ratio – 1.5
      Activated Partial Thromboplastin time:
      Patient – 47 seconds
      Control – 30 seconds

      Long prothrombin time and high INR can indicate:
      a lack of one or more blood clotting factors (factors I, II, V, VII, or X), or the depression of vitamin K dependent Factors VII,X and II.
      a lack of clotting factor activity,
      a vitamin K deficiency,
      liver disease,
      A long PT time can be caused by treatment with certain medications, such as heparin or warfarin (Coumadin), that are used to liquefy the blood and decrease clotting.
      Malabsorption or lack of intestinal colonization by bacteria (such as in newborns)

      Blood thinners as NSAID or Anticoagulants as heparin and Coumadin dosage given for people being treated to prevent the formation of blood clots such patients with heart diseases of artery who suffer from blood clotting criteria and patients with severe liver inflammation, or patients after surgery heparin may be given to decrease clotting.

      Anticoagulant dose must be adjusted gradually to avoid cute bleeding and according to the normal INR value and needed bleeding/clotting times as following:

      The normal blood protime and INR is: PT between 10–13 seconds is normal this corresponding to International normalized ratio (INR) between 1.0–1.4 as International normal range of INR
      For The warfarin (Coumadin) patients prothrombin time should be adjusted to be about 1.5 to 2.5 times the normal value (or INR values 2 to 3).
      For People with artificial heart valves
      Sometimes INR value should be higher than 3.0

      Coumadin/warfarin dose that rise INR values between 1.5- 2.5 usually up to 3.0 mg then it adjusted for every case later.
      Coumadin/warfarin dose needed to rise INR values higher than 3.0 usually starts from 5.0 mg and maybe go higher as needed for high bleeding demand as in heart valves surgeries.

      The best solution to lower INR and PT time is vitamin K intake physiologically by nutrients rich in Vit.K or using Vitamin K therapy.

      Example of medical condition may be as following:
      I developed a DVT in the right shoulder (same arm as IV during surgery) , go to the lab once / week. Last week my INR was 2.9 and I have been taken 5mg of warfarin. This week my INR was 6.6.

      The best time intervals to check for blood prothrombin time (Protime or INR) is 3 – 6 days according to half life time of Coumadin/warfarin in the blood (the time needed to fully metabolized usually it needs 2-4 days) with starting dose of 2.0 mg, cardiologist will request a lot of trial and error doses to fully understand and define the exact needed dose to make stabilization to patients, because some people metabolize Coumadin faster than others, after deciding the best dose and compatible time intervals patients can test for PT, and INR 1x/week, 2x/week, until reaching high stabilization it may be tested every 4 weeks.

      Make a dosage adjustment it should not be until the previous dose make change then you can increase/decrease the doses and after INR screening.

      Physician, Cardiologist, hematologist, Lab specialist, and cardiac liaisons nurses are the associated specialists to tell you the instructions and help you understand how Coumadin / Warfarin works to take the right doses and time intervals to carry out INR test.
      Best Home PT testing solution is the use of Home Testers or machines as test strips and home testing machines/meters like Coaguchek and Coaguchek-type machines to cover the high costs of repeated INR tests.

      The tests needed to diagnose bleeding and clotting status of the blood is The coagulation profile tests include a platelet count, a bleeding time, a coagulation time, a partial thromboplastin time, and a prothrombin time.
      Initial laboratory tests of coagulation disorder are Complete blood count (CBC), platelet count, peripheral smear, prothrombin time (PT), activated partial thromboplastin time (APTT), and possibly a bleeding time. If the lesions appear vasculitic, consider a sedimentation rate and C-reactive protein determination. Serum creatinine and urinalysis can be ordered to screen for renal involvement. In vasculitis, the laboratory findings are often nonspecific and a skin biopsy for histology is employed.

      If there is fever, blood cultures should be done. A bone marrow examination and bone marrow culture may be useful. If disseminated intravascular coagulation is suspected, a fibrinogen assay and estimation of fibrin degradation products should be done. Platelet function may be assessed by clot retraction tests. Spleen and liver scans and bone scans may be needed. A CT scan of the abdomen and pelvis may also be necessary. Skin, muscle, and even kidney biopsies are often done to complete the workup.

      Why is prothrombin time test not used for heparin - Biology

      The partial thromboplastin time (PTT also known as activated partial thromboplastin time (aPTT)) is a screening test that helps evaluate a person's ability to appropriately form blood clots. It measures the number of seconds it takes for a clot to form in a sample of blood after substances (reagents) are added. The PTT assesses the amount and the function of certain proteins in the blood called coagulation or clotting factors that are an important part of blood clot formation.

      When body tissue(s) or blood vessel walls are injured, bleeding occurs and a process called hemostasis begins. Small cell fragments called platelets stick to and then clump (aggregate) at the injury site. At the same time, a process called the coagulation cascade begins and coagulation factors are activated in a step-by-step process. Through the cascading reactions, threads called fibrin form and crosslink into a net that clings to the injury site and stabilizes it. This forms a stable blood clot to seal off injuries to blood vessels, prevents additional blood loss, and gives the damaged areas time to heal.

      Each part of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If the amount of one or more factors is too low, or if the factors cannot do their job properly, then a stable clot may not form and bleeding continues.

      With a PTT, your result is compared to a normal reference interval for clotting time. When your PTT takes longer than normal to clot, the PTT is considered "prolonged."

      When a PTT is used to investigate bleeding or clotting episodes or to rule out a bleeding or clotting disease (e.g., preoperative evaluation), it is often ordered along with a prothrombin time (PT). A healthcare practitioner will evaluate the results of both tests to help rule out or determine the cause of bleeding or clotting disorder.

      It is now understood that coagulation tests such as the PT and PTT are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, they can be used to evaluate certain components of the hemostasis system. The PTT and PT tests each evaluate coagulation factors that are part of different groups of chemical reaction pathways in the cascade, called the intrinsic, extrinsic, and common pathways.

      • The PTT is used to evaluate the coagulation factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK).
      • A PT test evaluates the coagulation factors VII, X, V, II, and I (fibrinogen).

      For more on this, see the article on the Coagulation Cascade.

      The PTT is used primarily to investigate unexplained bleeding or clotting. It may be ordered along with a prothrombin time (PT/INR) to evaluate the process that the body uses to form blood clots to help stop bleeding. These tests are usually the starting points for investigating excessive bleeding or clotting disorders.

      By evaluating the results of the two tests together, a healthcare practitioner can gain clues as to what bleeding or clotting disorder may be present. The PTT and PT are not diagnostic but usually provide information on whether further tests may be needed.

      Some examples of uses of a PTT include:

      • To identify coagulation factor deficiency if the PTT is prolonged, further studies can then be performed to identify what coagulation factors may be deficient or dysfunctional, or to determine if an antibody against a coagulation factor (known as a factor-specific inhibitor) is present in the blood.
      • To detect nonspecific autoantibodies (antiphospholipid antibodies), such as lupus anticoagulant these are associated with clotting episodes and with recurrent miscarriages. For this reason, PTT testing may be performed as part of a clotting disorder panel to help investigate recurrent miscarriages or diagnose antiphospholipid syndrome (APS). A variation of the PTT called the LA-sensitive PTT may be used for this purpose.
      • To monitor standard (unfractionated, UF) heparin anticoagulant therapy however, some labs now use the anti-Xa test to monitor heparin therapy. Heparin is an anticoagulation drug that is given intravenously (IV) or by injection to prevent and to treat blood clots (embolism and thromboembolism). It prolongs PTT. When heparin is administered for therapeutic purposes, it must be closely monitored. If too much is given, the treated person may bleed excessively with too little, the treated person may continue to clot.
      • Based on carefully obtained patient histories, the PTT and PT are sometimes selectively performed before a scheduled surgery or other invasive procedures to screen for potential bleeding tendencies.

      The PTT may be ordered along with other tests such as a PT when you have:

      • Unexplained bleeding or easy bruising
      • A blood clot in a vein or artery
      • An acute condition such as disseminated intravascular coagulation (DIC) that may cause both bleeding and clotting as coagulation factors are used up at a rapid rate
      • A chronic condition such as liver disease that may affect clotting
      • As part of an evaluation for lupus anticoagulant, anticardiolipin antibodies, and antiphospholipid syndrome, when you've had a blood clot or when a woman has had recurrent miscarriages
      • When you are switched from heparin therapy to longer-term warfarin (Coumadin®) therapy, the two are overlapped and both the PTT and PT are monitored until you have stabilized.
      • When you have a surgical operation scheduled you may have a PTT prior to surgery when the surgery carries an increased risk of blood loss and/or when you have a clinical history of bleeding, such as frequent or excessive nose bleeds and easy bruising, which may indicate the presence of a bleeding disorder.

      PTT results are typically reported in seconds.

      A PTT result that falls within a laboratory's reference interval usually indicates normal clotting function. However, even with a normal PTT result, mild to moderate deficiencies of a single coagulation factor may be present. The PTT may not be prolonged until the factor levels have decreased to 30% to 40% of normal. On the other hand, lupus anticoagulant may be present but may not prolong the PTT result. If the lupus anticoagulant (LA) is suspected, a more sensitive LA-sensitive PTT or a dilute Russell viper venom time (DRVVT) can be used to test for it. (See below for more about LA-sensitive PTT.)

      A prolonged PTT means that clotting is taking longer to occur than normal and may be due to a variety of causes.

      A prolonged PTT may be due to:

      • Underlying conditions that cause low levels of clotting factors, such as:
        • Liver disease—most coagulation factors are produced by the liver, thus liver disease may cause prolonged PT and PTT. However, PT is more likely to be prolonged than PTT.
        • Vitamin K deficiency—vitamin K is essential for the formation of several clotting factors. Vitamin K deficiencies are rare but can be caused by an extremely poor diet, malabsorption disorders, or prolonged use of certain antibiotics, for example. PT is more likely to be prolonged than is PTT.
        • Less common inherited clotting factor deficiencies:
          • von Willebrand disease (vWD) is the most common inherited bleeding disorder and it affects platelet function due to decreased von Willebrand factor. PTT is normal in most cases of vWD but can be prolonged in severe vWD.
          • Hemophilia A and hemophilia B (Christmas disease) are two other inherited bleeding disorders resulting from a decrease in factors VIII and IX, respectively.
          • Deficiencies of other coagulation factors, like factors XII and XI. Deficiency in XI can cause abnormal bleeding, but deficiency of XII is not associated with bleeding risk in the body.

          Results of the PTT are often interpreted with results of the PT in determining what condition may be present.

          PT resultptt resultCommon condition present
          ProlongedNormalLiver disease, vitamin K deficiency, decreased or defective factor VII, chronic DIC, warfarin or other vitamin K antagonist (e.g., brodifacoum in some cannabinoids)
          NormalProlongedHemophilia A or B (decreased or defective factor VIII or IX) or factor XI deficiency, von Willebrand disease (severe form), factor XII deficiency, or lupus anticoagulant present
          ProlongedProlongedDecreased or defective factor I (fibrinogen), II (prothrombin), V or X, severe liver disease, acute DIC
          NormalNormal or slightly prolongedMay indicate normal hemostasis however, PT and PTT can be normal in conditions such as mild deficiencies in other factors and mild form of von Willebrand disease. Further testing may be required to diagnose these conditions.

          A shortened PTT may be due to:

          • Disseminated intravascular coagulation (DIC)—in the early stages of DIC, there are circulating procoagulants that shorten the PTT.
          • Advanced cancer (ovarian, pancreatic, colon), except when the liver is involved
          • An acute-phase reaction: this is a condition causing extensive tissue inflammation or trauma that significantly elevates factor VIII levels. It is usually a temporary change that is not monitored with a PTT. When the condition causing the acute phase reaction is resolved, the PTT returns to normal.

          The LA-sensitive PTT (LA-PTT or PTT-LA) is a variant PTT, designed to evaluate the presence of lupus anticoagulant (LA), an antibody associated with clotting episodes and recurrent miscarriages. The LA-PTT uses a low phospholipid reagent that is optimized for detecting lupus anticoagulants and is therefore more sensitive to LA. The test is based on the principle that lupus anticoagulant binds to the phospholipids that are used as one of the reagents in the PTT test, causing an abnormally prolonged clotting time. For more on this, see the article on Lupus Anticoagulant Testing.

          Examples of other testing that may be done along with a PTT or in follow up to abnormal results include:

          • Platelet count – should always be monitored during heparin therapy to promptly detect any heparin-induced thrombocytopenia
          • Thrombin time testing – sometimes ordered to help rule out heparin contamination
          • Fibrinogen testing – may be done to rule out a low level of fibrinogen as a cause of a prolonged PTT
          • When an initial PTT is prolonged, a second PTT test is performed by mixing the person's plasma with pooled normal plasma (a collection of plasma from a number of normal donors). If the PTT time returns to normal ("corrects"), it suggests a deficiency of one or more of the coagulation factors in the person's plasma. If the time remains prolonged, then the problem may be due to the presence of an abnormal factor-specific factor inhibitor (autoantibody) or nonspecific inhibitor, such as lupus anticoagulant.
          • Coagulation factor tests – these measure the activity (function) of coagulation factors. They can detect reduced levels of the protein or proteins that don't work properly (have reduced function). Rarely, the antigen level (quantity) of a coagulation factor may also be measured.
          • Dilute Russell viper venom test (DRVVT) – a test that may be done if the presence of lupus anticoagulant is suspected (See the page on Lupus Anticoagulant Testing for more on this.)
          • von Willebrand factor – sometimes ordered to help determine if von Willebrand disease (severe form) is the cause of a prolonged PTT.

          In a few situations, it is not.

          1. When very high doses of heparin are used, as may occur during open-heart surgery, the PTT loses its sensitivity it will not clot. At this intense level of anticoagulation, the activated clotting time (ACT) can be used as a monitoring tool.
          2. Some hospitals now monitor standard (unfractionated) heparin therapy using the chromogenic anti-factor Xa test in lieu of PTT.
          3. Low molecular weight heparin (LMWH) is a fast-acting form of heparin often used in the treatment of conditions such as deep vein thrombosis (DVT) prevention. Though generally not requiring monitoring, it must be monitored using the anti-factor Xa test because LMWH typically does not prolong PTT.
          4. For people with lupus anticoagulant and clotting and who are being treated with heparin, the PTT is not reliable thus the anti-factor Xa test must be used to monitor their heparin therapy.

          The PTT is not something you can change through lifestyle changes (unless you have a vitamin K deficiency). It is a reflection of the integrity of your clotting system. If your PTT is prolonged due to acquired factor deficiencies, then addressing the underlying condition may bring the results to near normal levels. If they are prolonged due to a temporary or acute condition, they should return to normal on their own when the acute condition is resolved. Inherited coagulation abnormalities or deficiencies should be closely monitored and may be treated with frequent replacement infusions of the missing clotting factor.

          Two anticoagulants often used, low molecular weight heparin (LMWH) and danaparoid, may not prolong the PTT and, if indicated, should be monitored using the heparin anti-factor Xa assay.

          Several factors can affect results of a PTT and the interpretation of test results: