Evaluation of between-, within- and day-to-day variation of coagulation measured by rotational thrombelastometry...

The aim of this study was to assess the circadian variation and the between- and within-subject variation in 10 healthy subjects over a period of 8 weeks by ROTEM^®. We further evaluated the influence of elevated body mass index and the effect of low molecular weight heparin and antithrombin on clot formation.

Citrated blood samples were analysed in the NATEM^® test system. The clotting time (CT), clot formation time (CFT), maximum clot firmness (MCF) and the maximum lysis (ML) were assessed.

Duplicate measurements showed that 23% of the CT and 31% of the CFT measurements had a coefficient of variation (CV) greater than 10%. The within-subject CV was 16% for the CT and 30% for the CFT. The MCF was fairly constant (6%), whereas ML showed more variation (18%). The between-subject CV was 6% for the CT and 20% for the CFT. Analytical variability was improved by summing up CT and CFT. Compared to morning values, CT, CFT and the sum of CT + CFT were shortened in the afternoon. High body mass index was associated with faster clotting. High concentrations of antithrombin had similar effects on clot formation as 0.2 IU/ml of enoxaparin.

To overcome the influence of diurnal variation, we recommend obtaining blood samples at specified times in the morning. The within-subject variation should be taken into account, when serial measurements of drug effects are required.