BOF: Clotting (Laboratory Assays)

Which ONE of the following statements is true of the international normalised ratio (INR) and the International Sensitivity Index (ISI)?

a) Each laboratory should use the reagent thromboplastin manufacturer’s recommended ISI in order to determine the INR

b) Ideal ISI values are greater than 1.5

c) INR is the standard means of reporting prothrombin time (PT) in any hospitalised patient

d) Mean prothrombin time used in deriving INR should be determined from at least 20 blood samples derived from a local population of people not on warfarin

e) The ratio is calculated by the fo

rmula: INR = (Patient time / Arithmetic mean prothrombin time)^ISI

Answer:

d) Mean prothrombin time used in deriving INR should be determined from at least 20 blood samples derived from a local population of people not on warfarin

Explanation:

In order to standardise the results of the INR across laboratories worldwide and different thromboplastin reagents, a WHO standard thromboplastin is used, against which all other thromboplastins are measured. This relation to the standard thromboplastin is calculated as the ISI. This is given by the manufacturer of each reagent, however individual laboratories should calculate their own using standard plasma and calibration kits to ensure local accuracy. Good thromboplastin reagents should have an ISI close to 1. INR should really only be used to report the PT in patients on warfarin, but this is often not adhered to in practice. The GEOMETRIC mean prothrombin time should be calculated using a local population appropriate to each laboratory.

Reference:

Perry D & Todd T. Practical Haemostasis.com: ISI & INR