Reference intervals for prothrombin time and activated partial thromboplastin time derived from black African blood donors at a university teaching hospital in Nairobi, Kenya

Date of Award


Document Type


Degree Name

Master of Medicine (MMed)

First Supervisor/Advisor

Dr. Nancy Okinda

Second Supervisor/Advisor

Dr. Geoffrey Omuse

Third Supervisor/Advisor

Dr. Riyat Malkit


Pathology (East Africa)


Introduction: Prothrombin time (PT) and activated partial thromboplastin time (APTT) are important coagulation screening tests that give an insight into how well the extrinsic and intrinsic arms of the coagulation cascade are performing. For correct interpretation of laboratory results and avoidance of errors and misdiagnosis, population specific RIs are most appropriate. In a healthcare setting, it is up to the laboratory to establish RIs for use for the particular analytes that they run, or at least verify the RIs availed by the manufacturer of the reagents for a particular analyte.

Objectives: This study aimed to establish reference intervals (RIs) for both PT and APTT at the Aga Khan University Hospital, Nairobi (AKUH-N) laboratory using black African blood donors. In doing so the study also looked into factors that might lead to differences in the intervals; gender, age and blood group

Methods: As per clinical and laboratory standards institute (CLSI) guidelines, at least 120 male and 120 female black African donors were recruited, and samples collected for PT and APTT, after informed consent. Data on age, gender, blood group, PT and APTT results were entered into Microsoft Excel® spread sheet for analysis.

Data Analysis: Analysis of the data was performed using SPSS® and reference value advisor v2.1. CLSI guidelines for determining reference intervals were used. RIs derived from the study were compared to the RIs provided by the manufacturer of the platform using the reference change value (RCV). Mann–Whitney U test was performed to determine differences between groups. Pearson’s correlation was run to see if there was any correlation between PT/APTT and age.

Results: A total of 258 study participants were enlisted. The PT reference intervals were 10.50–13.30 seconds while the APTT reference intervals were 24.13-35.10. Based on reference change value (RCV), both were significantly higher than manufacturer provided intervals. There was a statistically significant lower PT and APTT in females as compared to males (U=3971 p<0.0005; U=5924.5 p 0.004), while blood group O participants had a significantly higher APTT compared to non-O participants (U=5613 p 0.004). Our study also found a weak negative correlation of PT with age (Pearson coefficient -0.127, p 0.049).

Conclusion: We report a reference interval for PT and APTT for black African adults that is significantly different from the manufacturer provided RIs and recommend adoption of the same by our laboratory. We further recommend stratification of RIs according to gender due to the significant difference in levels.

This document is available in the relevant AKU library