1. What is the principle behind the Prothrombin Time test (PT) Prothrombin Time test (PT) also be called the INR test (WebMD Medical Reference, 2008) is a blood test which measures the period it takes blood to clot.. The principle behind the Prothrombin Time test is that in the body, the clotting process involves a series of sequential chemical reactions called the coagulation cascade. The PT test evaluates the integrated function of the coagulation factors that comprise the extrinsic and common pathways of the coagulation cascade, including factors I (fibrinogen), II (Prothrombin), V, VII and X. It evaluates the body’s ability to produce a clot in a reasonable amount of time and, if any of these factors are deficient, the PT will be prolonged.
2. Why is this test effective for monitoring oral anticoagulant therapy
Prothrombin test is most common to use for monitoring in oral anticoagulant therapy Warfarin and related coumarins. The PT responds to reduction of three of the four vitamin K-dependent procoagulant clotting factors (II, VII, and X). This PT test gauges the time or period of reaction of Plasma to Thromboplastin and Ca++ that results to Fibrin Clot.
3. What is the difference between reporting PT results as INR or Prothrombin Index Quick activity (PI%)
The INR value is calculated from the patient’s coagulation time. However, the ISI value is included in the calculation. The manufacturer calculates the ISI value (International Sensitivity Index) by comparison to an international standard thromboplastin. This way, the different sensitivities of reagents available in the market are compensated.
. INR results are calculated in seconds ,according to the formula:
INR = (samples/normals)ISI
Normal = 1
Therapeutic Range = 2 – 4.5
While in Prothrombin Index. P.I, a more widely used expression, the result is not quoted in seconds but as a percentage of normal, calculated by
Normal Control 12 secs. xl00 =50%
Patient Prothrombin Time 24 secs.
Normal = 90 – 110%
Therapeutic Anticoagulant Range = 40 – 60%
4. Calculate the mean and standard deviation for the class results.
INR PI (%)
Mean 1 93.16
SD 0.1982 27.18
5. Comment on your result compared with that of the other groups and overall class results.
Our result of INR 1.1 and PI% 90.9 is comparable and consistent with results of all other groups which ranges between 0.9 and 1.1 and it is also quiet comparable and consistent with the class mean result for INR and PI index.
6. Examine the ten results listed on page 4. These results were obtained when single quality control plasma (taken from the same bottle) was tested ten times. Are there any interesting features apparent with these results Do you think these results are acceptable
The mean INR value for the results is 3.07,two readings i.e 2.6 and 2.0 are quiet distant from the mean and comprise 20% of the readings ,which is more than the acceptable 15% variation(Leon Poller and others 2006),therefore we can say that the instrument might need to be looked into for improved calibration before accepting the results.
Jackson, C.M., Esnouf, M.P. And Lindahl, T.L. (2003) A critical evaluation of the prothrombin time for monitoring oral anticoagulant therapy. Pathophysiology Haemostasis and Thrombosis Journal, 33(1):43-51.
Leon Poller, Michelle Keown, Saied A. Ibrahim, Felix J.M. van der Meer, Anton M.H.P. van den Besselaar (2006).Quality Assessment of CoaguChek Point-of-Care Prothrombin Time Monitors: Comparison of the European Community-Approved Procedure and Conventional External Quality Assessment Clinical Chemistry 52: 1843-1847.
Parks, R. (2008) Hematology. WebMD Medical Reference. 15.
Rinsho, B. (2002) Prothrombin time and its standardization. National Center for Biotechnology Information, Pubmed, 50(8):779-85.
Wochenschr, S. M. (1998) Comparison of quick/INR values of whole capillary blood (coaguchek plus) and venous citrate plasma in patients with and without oral anticoagulation. National Center for Biotechnology Information, PubMed, 128(44):