OBJECTIVES: Biological variation (BV) data obtained in a standardized way is valuable to assess the analytical requirements and the utility of a reference interval. Our study aimed to determine the short-term BV of thrombophilia (protein S, protein C, activated protein C resistance (APCR) and factor VIII) and hemophilia (factors VIII, IX and XI) parameters in plasma. Coagulation factors V and XII were also evaluated. Based on the obtained data, we assessed analytical performance specifications for the parameters. Finally, we intended to provide a robust tool for comparison of serial measurements of factors V, VIII, IX and XI. METHODS: A blood draw was performed weekly in 19 apparently healthy Caucasian adults for five weeks at Saint-Luc University Hospital (Brussels, Belgium). Parameters were measured in duplicate. BV components were calculated with a nested analysis of variance after exclusion of outliers. RESULTS: The analytical coefficient of variation (CV) varied from 1.5 to 4.6%, the within-subject CV from 1.6 to 8.9% and the between-subject CV from 3.8 to 24.1%. All parameters showed high individuality. For most parameters, the analytical goal was met with our assays. Reference change values (RCV) of -16.7% to +20.0%, -20.7% to +26.0%, -15.3% to +18.1% and -13.1% to +15.1% were obtained for factors V, VIII, IX and XI respectively. CONCLUSIONS: All studied parameters were highly individualized. The assessment of BV data can guide setting analytical goal specifications. Comparison of serial measurements in the follow-up of patients suffering from hepatic failure or mild hemophilia is facilitated by evaluation of the RCV.