Correlation between PT/INR and APTT in Patients on Warfarin: A Study at a Tertiary Care Teaching Hospital

Background: Warfarin prolongs coagulation by inhibiting the synthesis of biologically active forms of coagulation factors and regulatory factors, which are dependent on vitamin K. The Prothrombin Time (PT) and International Normalized Ratio (INR) are routinely used to monitor warfarin therapy in clinical settings. In addition to PT, APTT is also prolonged by warfarin therapy due to the effect of warfarin on intrinsic pathway. In this study we aimed to evaluate the distribution of PT/INR, distribution of APTT and correlation between INR and APTT in a cohort of patients on long term warfarin.

Materials and Methods: Seventy four (74) patients on long term warfarin therapy were recruited from University Haematology clinic of Colombo South Teaching Hospital. PT and APTT were performed in the routine citrated blood samples during their regular clinic visits using semi-automated 4 channel CA-104 analyzer. Data were analyzed statistically by using SPSS.

Results: Among the recruited 74 patients, 36 were females and 38 were males. In this cohort, PT was normally distributed. The distribution pattern of APTT deviated from the normal standard pattern due to some exceptional cases of females. There was no significant correlation between INR and warfarin dose. The statistical results of the study established that there was a good (strongly positive) linear correlation between the APTT and INR since Pearson correlation coefficient equals to 0.799 (p=0.000).

Conclusion: Our study concluded that in addition to the elevation of PT/INR and APTT by warfarin, there is a good linear correlation between elevation of INR and APTT.