Laboratory Wise Variations in Prothrombin Time - INR - A Cross Sectional Study in Trivandrum, Kerala

Author(s): Shahana Jahan Kulathinte Meethal1, Ravikrishnan Jayakumar2, Suresh Kumar Sreenivasan3, Shaffeek Abdul Majeed4, Indira Kariveettil5

Prothrombin time (PT) is routinely used as a test of coagulation. Thromboplastin
is the key ingredient in the reagent for this test. Prothrombin time international
normalized ratio (INR) readings can vary according to the thromboplastin used in
the reagent. The composition of thromboplastin reagents can influence the
sensitivity of each batch of reagents. Various thromboplastin reagents having
different international sensitivity index (ISI) values are available now. This study
was intended to evaluate the effect of different thromboplastins on INR reading
for mitral valve replaced patients under stable oral anticoagulant therapy.
The study was conducted on the citrated plasma received from the mitral valve
replaced patients having stable international ratio between 2 to 3 for three months.
62 samples were collected from the clinical pathology laboratory, Govt. Medical
College, Trivandrum. Each sample was tested with different thromboplastin
reagents having international sensitivity index 1.0, 1.1 and 1.6 by measurement
of the prothrombin time and conversion into international normalized ratio. The
INR obtained from the thromboplastin with international sensitivity index 1.0 was
considered as the standard. INR results obtained from samples tested with
thromboplastin reagents with ISI 1.1 and 1.6 were compared with the standard
by using analysis of variance (ANOVA) and Dunnett’s post hoc tests.
Sixty-two samples were tested with the thromboplastin reagent having ISI – 1.0,
the mean INR is 2.42, for ISI – 1.1 mean INR value is 2.53 and for ISI 1.6, the
mean INR value is 3.19. While comparing the mean value of INR for different
reagents using ANOVA, the F value was 14.86, which was significant. P value less
than 0.01. In the Dunnett post hoc test, the P value of difference between ISI
1.0/1.6 was < 0.01. Between ISI 1.1/1.6 also the P value is < 0.01. Both of these
were significant. The P value of difference between the reagents having ISI 1.0
and 1.1 is 0.838 which denotes no significant difference.
In conclusion, the thromboplastin reagent with ISI 1.0 or nearest to 1.0 is most
desirable for accurate INR report.