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Dear Sir,
Prothrombin time (PT) is the measure of clotting velocity after addition of Thromboplastin – either purified or recombinant thromboplastin/tissue factor like Innovin - and calcium ion to citrated plasma, and this provides a strong stimulus of the activation of factor VII and the common pathways.
Prothrombin time (PT) determination is an excellent index of Prothrombin adequacy, and it can vary significantly depending on the type of Thromboplastin type.
Prothrombin time (PT) is commonly increased in liver diseases because the liver is unable to manufacture adequate amounts of clotting factors or because of the disruption of bile flow results in inadequate absorption of vitamin K from the intestine.
Because of Prothrombin time response to vitamin K administration, PT helps in differentiation of some intrahepatic diseases with decreased synthesizing capacity from extrahepatic obstruction with decreased absorption of fat – soluble vitamins.
A marked prolongation of the Prothrombin time indicates severe diffuse liver disease and a poor prognosis.
The PT is prolonged in vitamin K deficiency and in liver diseases characterized by decreased synthesis of Prothrombin, and the levels of the vitamin K – Dependent factor must be reduced by more than50% –60% to prolong PT. Vitamin K deficiency also results in in the prolongation of Partial Prothrombin time or also known as Activated Partial Thromboplastin Time (PTT), but the Thrombin time is within the reference interval.
The PT provides a measure of the extrinsic and common pathways. Compared with the PTT, it is more sensitive to deficiencies of Vitamin K- dependent liver factors (II, VII, IX and X). As a screen for any single factor deficiency, the PT is most sensitive to a reduced level of factor VII. When the PT is greater than2 times of control, factor levels are most likely below20% of normal.
When the PT is used to monitor a patient receiving an oral Warfarin, the sensitivity of thromboplastin on the particular clotting instrument must be taken into account in interpreting the prolongation of the PT. This Lab. situation has led to the development of standardized method of expressing the PT prolongation as an International Normalized Ratio (INR).
The PT can also be slightly prolonged by large amounts of Heparin in circulation; more rarely by circulating inhibitor; and either a reduced level of Fibrinogen or the appearance of large amounts of abnormal Fibrinogen molecules or Fibrin fragments in circulation.
The PT may be prolonged if the whole blood specimen is stored too long prior to plasma separation, owing to degradation of coagulation proteins of whole blood.
The Activated Partial Thromboplastin Time (PTT) measures both the intrinsic and common coagulation pathways (XII, XI, IX, VIII, X, V and II). For this assay, citrated plasma is activated with a contrast surface material, such as Kaolin, together with calcium ion and phospholipid. Unlike the INR, the PTT normal range may vary widely between labs. Thus, the clinician must be familiar with the normal range of clotting time in different labs.
The PTT can be prolonged by deficiencies of any factors in the intrinsic or common pathway. By the presence of intrinsic inhibitor, or because of fibrinogen abnormality, with the later due to either to reduced level or function. In order to significantly prolong the PTT, single factor deficiencies must be relatively severe, that is, below30% -40% of normal. Multiple factor deficiencies of somewhat lesser severity will also prolong the PTT.
Although the PTT is much less sensitive than the PT to low levels of vitamin K – Dependent factors, the PTT is more sensitive to the presence of circulating anticoagulant such as Heparin and the Lupus Anticoagulant.
- To identify a circulating anticoagulant, the lab can repeat the PTT with1:1 mix of patient and normal plasma. Although the admixture of normal plasma will:
a- Completely correct a prolonged PTT secondary to a factor deficiency (i.e. factor VIII OR IX deficiency).
b- Not fully correct (or shorten) the PTT that is prolonged due to high titer antibody, also known as a "circulating anticoagulant".
A prolonged PTT resulting from unfractionated Heparin in the sample, either because of therapeutic anticoagulation, or inadvertent contamination by drawing the sample through an indwelling well, can be determined by:
- Mixing the patient plasma with the substance such as Heparinase or polybrene; this procedure will:
a- Neutralize the Heparin and
b- Correct the PTT to within or near the normal reference range.
c- Thus, a prolonged PTT in hospitalized patient should be investigated first with Heparin neutralization, followed by a mixing study if there is no evidence for heparin in the sample.
- Failure to correct the prolonged PTT with plasma requires subsequent evaluation for a Lupus Anticoagulant or a factor inhibitor such as seen in patients with Hemophilia.
Best Regards,
Lubna Al-Sharif
pt protrombin time - ptt parital tromboplastin time- the first test exam the external factors of coagolation espicially when the patient take warferin and drugs took by mouth -the second test exam the internal factors of coagolation espicially when the patient took heparin the two tests also detect the diseases of coagolations like the lake of factors in internal and external ways of coagolation
1. PTT is ‘partial thromboplastin time’ and PT is ‘prothrombin time.’
2. The PT measures the extrinsic coagulation pathway by calculating the prothrombin ratio along with the international normalized ratio.
3. The PTT is a measurement of the intrinsic coagulation pathway and the common coagulation pathway.
4. The clotting factors like I, II, V, VII and X are looked into in prothrombin time. The clotting factors that are noted in PTT are I, II, V, VIII, IX, X, XI and XII.
5. The PT also determines the warfarin levels and the vitamin K position. The partial thromboplastin time measures the heparin level in the body.
6. The normal PT value is11 to16 seconds. The normal PTT value is25 to39 seconds.
Prothrombin, or factor II, is one of several clotting factors produced by the liver. Prothrombin time (PT) measures the presence and activity of five different blood clotting factors (factors I, II, V, VII, and X). This test measures the integrity of the extrinsic and common pathways of coagulation. The prothrombin time (PT) measures the clotting time from the activation of factor VII through the formation of the fibrin clot. Adequate amounts of vitamin K are needed to produce prothrombin. Warfarin (Coumadin) is sometimes prescribed as a "blood thinner" because it is an effective vitamin K antagonist (blocks the formation of vitamin K). The activated partial thromboplastin time (PTT) measures the function of several other clotting factors, found in the intrinsic and common blood clotting pathways. It measures the clotting time from the activation of factor XII through the formation of the fibrin clot. PTT prolongations are caused by factor deficiencies (factors VIII, IX, XI, and/or XII), or inhibitors (most commonly, heparin). Heparin prevents clotting by blocking certain factors in the intrinsic pathway.
PT is used to monitor treatment with warfarin (Coumadin). Once warfarin is discontinued, it usually takes several days to clear it from the system. The PTT test allows the provider to check that there is enough heparin in the blood to prevent clotting, but not so much as to cause bleeding. Once heparin is discontinued, its blood-thinning effects usually only last a few hours. The greater the PT/PTT values, the longer it takes the blood to clot. HIGH PT/PTT = risk for BLEEDING. LOW PT/PTT = risk for blood clots/ stroke. The INR stands for International Normalized Ratio. The INR is calculated from the PT and is intended to allow valid comparisons of results regardless of the type of PT reagent used among different laboratories (INR = [patient PT / mean normal PT]). The INR is a method of standardizing the PT for coumadin anticoagulation. Before the INR, different labs using different reagents had different controls and widely differing PT value ranges. An INR of1 means the blood clots "normally" for that pt. The greater the INR, the longer it takes the blood to clot. The warfarin (Coumadin) dosage for people being treated to prevent the formation of blood clots is usually adjusted so that the prothrombin time is about1.5 to2.5 times the normal value (or INR values2 to3). A patient may take warfarin to anticoagulate for atrial fibrillation (target INR around2.5) or for a mechanical heart valve (target INR3). An INR of3 means the blood takes about three times as long to clot compared with the normal value for that pt.
