The phosphatidylcholine exchange protein from beef liver catalyzes the exchange of phosphatidylcholine between single bilayer liposomes (Hellings et al. (1974), Eur. J. Biochem. 47, 601). A model has been proposed which describes the kinetics of this exchange. Steady-state equations have been derived from the model and have been used for the derivation of the theoretical rate equation. Computer analysis shows a good fit with the experimental results. It follows from the analysis that the apparent dissociation constant of the exchange protein-liposome complex decreases with an increasing phosphatidic acid content of the liposomes. This suggests that in this model system it is the phospholipid composition of the membranes involved that regulates the amount of exchange protein available to function as a carrier of phosphatidylcholine.

SummaryTwo commercial rabbit tissue thromboplastins were calibrated against the International Reference Preparation for rabbit thromboplastin (coded RBT/79) by one laboratory using the manual technique, a semi-automatic electro-mechanical coagulometer and three different automatic photo-optical instruments. The calibration of the two reagents was performed in three and two different exercises, respectively, and showed good reproducibility of the procedure. The purpose of calibration is providing a formula for the assessment of the International Normalized Ratio (INR) for patients receiving oral anticoagulants. The World Health Organization (WHO) model for thromboplastin calibration leads to the equation INR = RISI, in which R is the prothrombin time ratio and ISI the International Sensitivity Index of the calibrated thromboplastin/instrument system. This equation was adequate for one reagent, but not for the other when it was used in combination with the four instruments. At therapeutic intensities of anticoagulation, the deviation from the WHO-model observed for the second reagent was clinically insignificant. The WHO model was fully adequate when the second reagent was used with the manual technique. For both thromboplastins, there were statistically significant differences in ISI between the four instruments. The largest difference-amounting to approximately 10%-was observed between two photo-optical instruments. The order of instruments with increasing ISI was the same for the two reagents. It is recommended that thromboplastin manufacturers specify the instruments used for calibration of their reagent.

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Summary The new CoaguChek XS system is designed for use in patient self testing with a measuring range from 0.8 INR up to 8.0 INR, which has been calibrated against the mean INR of rTF/95 and ERM‐AD149. This study was performed to confirm the correct INR results received from two routinely manufactured lots of test strips when compared with the international reference preparations (IRP) rTF/95 and ERM‐AD149. At one study site capillary and noncitrated venous whole blood samples from 20 normal donors and 62 anticoagulated patients were applied to two test strip lots of the new system in duplicate. Additionally blood was collected in citrate tubes, processed to plasma, and PT results were obtained using rTF/95 and ERM‐AD149 by the manual tilt tube method. Method comparisons of the INR results of the CoaguChek XS system vs. the mean INR of the IRP demonstrated a mean relative bias of −0.02% to −0.4%, mean absolute relative deviations of 6.4–9.6%, and accuracy observing >95% of CoaguChek XS INR within limits of ±14% to ±21.5% to the mean INR of the IRP. The results of the study confirm the successful calibration of two lots of the new CoaguChek XS system, demonstrate the validity of the calibration concept and prove the accuracy of the new system in comparison with the IRP. Clinical decisions in oral anticoagulation therapy may be reliably made upon the INR results of the new system.

Summary. A multicentre modified World Health Organization (WHO)-type international sensitivity index (ISI) calibration has been performed at 10 European Concerted Action on Anticoagulation (ECAA) national laboratories using non-citrated whole-blood on two point-of-care test (POCT) prothrombin time (PT) monitor systems, CoaguChek Mini and TAS PT-NC, using single lots of test cards/strips. The relevant species (human and rabbit) WHO international reference preparations (IRPs) were tested with the manual PT technique on citrated plasma from the same blood donations. The ISI was calculated from the slope of the orthogonal regression line relating log PT (POCT) to log PT (IRP). The mean ISI of the CoaguChek Mini system was 1AE75 and 1AE13 with the prothrombin time non-citrated Thrombolytic Assessment System (TAS PT-NC). With the CoaguChek Mini system, seven out of 10 calibrations exceeded the current 3% WHO recommended limit for the coefficient of variation (CV) of the slope with conventional PT testing, whereas with the TAS PT-NC system, it was eight out of 10. All the POCT calibrations had a CV of the slope < 5%. It is suggested that this level of precision be adopted as the limit of acceptability of calibration of these monitor systems. In these circumstances, the modified WHO-type ISI calibration appeared to be satisfactory for the POCT whole-blood monitors.