Usefulness of high-concentration calcium chloride solution for correction of activated partial thromboplastin time (APTT) in patients with high-hematocrit value

IntroductionPseudoprolongation of activated partial thromboplastin time (APTT) is a serious problem in anticoagulation therapy for patients with high hematocrit, such as cyanotic congenital heart diseases. APTT pseudoprolongation occurs when APTT assay is performed using routinely used vacuum sampling tubes containing citrate. Because the plasma fraction is small in high-hematocrit blood, the prescribed volume of citrate would be excessive for APTT assay, resulting in prolongation of clotting or APTT pseudoprolongation. CLSI — The Clinical and Laboratory Standards Institute (formerly NCCLS) method is the established method to correct the pseudoprolongation. However, the CLSI method needs repeated blood drawings and time-consuming, complicate procedures. Thus, alternative simple method is desired.MethodWe examined whether APTT pseudoprolongation would be prevented by the increase in free calcium ion concentration by using high-concentration calcium chloride solution for the assay. Blood samples were obtained from 15 patients with high hematocrit (65 ± 6%) who had cyanotic congenital heart disease.ResultConventional APTT assay using 0.025 mol/L calcium chloride solution gave greater APTT compared with the CLSI method (51.7 ± 11.8 vs. 34.6 ± 4.7 s, p < 0.001). However, when 0.035 mol/L calcium chloride solution was used, APTT (36.6 ± 5.8 s) was similar to that obtained from the CLSI method. There was a good correlation in APTT values between high-calcium chloride solution method and the CLSI method (the slope = 0.57, r2 = 0.49).ConclusionHigh-calcium chloride solution method is useful to correct APTT pseudoprolongation. Because of the simplicity and the need of a single blood drawing, this method would reduce the burdens of not only patients but also clinical laboratory.