Laboratory suggestions: Improving the sensitivity of the Fletcher factor assay.

The prolonged activated partial thromboplastin times observed with Fletcher factor (prekallikrein)-deficient plasmas become nearly normal after the plasma is incubated with contact surfaces or with an activator. Those shorter times may lead to relatively flat reference curves for Fletcher factor assays under certain conditions. The authors examine in detail the effect of incubation time upon the sensitivity of Fletcher factor assays. Four different surface activators were used in the tests to determine the optimal activation for each type. An incubation time of 1 min produced maximal sensitivity with each surface activator tested. All reagents remained stable for at least an hour at 37.5 C.