Effect of a common reference plasma on the inter-laboratory variation of the measurement of total and free protein S: a collaborative study of the Dutch Working Group on Haemostasis Laboratory Diagnosis

Abstract

The comparability of test results for protein S between laboratories is hampered by a high inter-laboratory variability. The effect of the use and type of common reference plasma on the inter-laboratory variability of the total and free protein S measurement was evaluated. The results of 10 plasma samples measured against a centrally distributed frozen plasma and a centrally distributed lyophilized plasma were compared with those of various local reference plasmas regularly used by the 11 participating laboratories. The mean intra-assay coefficient of variation for total protein S in each laboratory varied from 3.8 to 12.8% (mean intra-assay CV of all laboratories: 7.4 &#45 2.3%); for free protein S this range was 3.1 to 13.3% (mean intra-assay CV of all laboratories: 6.6 &#45 2.7%). We confirmed the high inter-laboratory coefficient of variation (mean &#45 SD) with the several local reference plasmas for both total protein S (13.4 &#45 5.6%; n = 10) and free protein S (17.1 &#45 7.5%; n = 11). For total protein S, the inter-laboratory CV was reduced to 11.5 &#45 4.8% (p = 0.005) by using a common frozen reference plasma, while it was increased to 16.8 &#45 3.4% (p = 0.022) using a common lyophilized reference plasma. For free protein S, these values decreased only statistically significantly for the common lyophilized reference plasma, to 15.1 &#45 6.0% (p = 0.008). For free protein S, the dilution factor used was identified as a factor influencing the inter-laboratory variability. This study shows that, for both types of protein S measurements, using one frozen reference plasma shows a slight decrease in inter-laboratory variability, while a common lyophilized plasma shows inconsistent results. It is concluded that further investigation is necessary to examine other sources of variability to increase the comparability of laboratory results for both total and free protein S.

• Calibration
• Coefficient Of Variation
• Immunological Assay
• Interlaboratory
• Intra-LABORATORY