Clotting time by free oscillation rheometry and visual inspection and a viscoelastic description of the clotting phenomenon.

An automated procedure for determination of clotting time in whole blood was validated by direct comparison with the reference method, visual clotting time determination. The procedure was based on a 10 Hz free oscillation rheometer (FOR) of our design, the ReoRox 4. Recalcified citrated blood samples (n = 30), clotting in the range 4 to 20 min, were used in the validation. Every 30 s of the analysis, as the change in stiffness (deltaG*) of the sample was monitored by FOR, the sample cup was shortly removed from the FOR and its contents inspected for first signs of clotting, i.e. visual clotting time determination. Various FOR clotting criteria were attempted. Best correlation to visual clotting time was found when deltaG* reached 0.01 Pa, which yielded linear regression slope, intercept and r2 of 0.98, 0.09 min and 0.98, respectively. For comparison, six plasma samples were analyzed in the same way and gave almost the same results. The accuracy of the FOR determinations was checked by also analyzing, in parallel, portions of the sample with a conventional oscillation rheometer, a Bohlin VOR. The rationale is given for preferring deltaG* over G* as a FOR monitoring function in coagulation tests and for including median filtration of the primary FOR data. An extension of the FOR theory to include deltaG* and evidence in support of inhomogeneous blood clotting are also given.