Calculation of Mean Normal Prothrombin Time (MNPT)

• Mean Normal Prothrombin Time- It is geometric mean of PT of reference sample group. MNPT is the denominator for calculation of international Normalized Ratio (INR)

• With each new lot of PT reagent, MNPT is derived for calculation of INR

• Method for calculation of MNPT

1. Collect citrated plasma of apparently healthy individuals (20 donors)(i.e. collect blood from 20 healthy individuals in citrate vacutte and centrifuge them at 1500 g for 10 min to obtain platelet poor plasma.
2. Perform PT of each Sample. E.G. the PT value for different sample s X1,X2, X3, X4, X5, X6, X7, X8, X9, X10, X11, X12, X13, X14, X15, X16, X17, X18, X19, X20
3. Take out geometric mean of these 20values.
4. geometric mean(y)= (X1,X2, X3, X4, X5, X6, X7, X8, X9,…………………………….X20)1/20
5. This geometric mean (y) is Mean Normal Prothrombin Time
6. Any PT Value is reported in the form of INR.

INR= (PT of test/MNPT)ISI Where ISI is provided by the company providing the PT reagent

1. To establish the normal reference range of PT, after taking out the arithmetic mean, calculate standard Deviation (SD)

Reference:

1. Reference Manual of STA compact (STAGO)
2. Practical hematology (Dacie and Lewis) - Eleventh edition; 2012
3. Reagent insert of STAGO reagent (Neoplastin CI Pl