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The Variability of Measurements of the Prothrombin Time Ratio in the National Quality Control Trials: a Follow‐up Study
Author(s) -
Leck Ian,
Gowland E.,
Poller L.
Publication year - 1974
Publication title -
british journal of haematology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.907
H-Index - 186
eISSN - 1365-2141
pISSN - 0007-1048
DOI - 10.1111/j.1365-2141.1974.tb06677.x
Subject(s) - prothrombin time , medicine , thromboplastin , external quality assessment , clinical trial , pathology , coagulation
S ummary . British hospitals which use the one‐stage prothrombin time tat my participatc in one of two schemes. Those in the Routine Supply Scheme use a standardized tissue thromboplastin, the Manchester Comparative Reagent (MCR). Those in the National Referencc Scheme use other reagents routinely, but calibrate thcm against the British Comparative Thromboplastin (BCT) which consists of batches of thc MCR which have been checked for accuracy by a group of expert Monitoring Centrcs selected from the hospitals in each scheme. The performance of hospitals in both schemes is monitored by trials in which they are asked to measure prothrombin time ratios, using BCT, on lyophilized plasma preparations which are sent to them. Four trials are reported here. In one, a single plasma preparation was used. In each of thc other three, samples of three plasmas were sent. There was considerable variation in the ratios reported by different hospitals for each plasma. In the trials involving three plasma samples, a marked tendency was demonstrated for hospitals which obtained a particularly high or low reading for one plasma to deviate in the same direction when measuring other plasmas in the same trial, but this was not found when readings from different trials (separated by several months) were compared. A breakdown of the results by type of hospital suggested that those obtained by the participants in the Routine Supply Scheme, and especially by the Monitoring Centres m this group, were more accurate than those of the National Reference Scheme hospitals, but within each group no persistent differences in accuracy between hospitals and no general improvement between the earlier and later trials could be demonstrated. Confidence limits estimated com the results of all mals suggest that even in Routine Supply Scheme hospitals, the chances of having a true prothrombin time ratio outside what is usually quoted as the therapeutic range (1.8‐3.0) should be regarded as higher than I in 20 for every patient whose reported ratio @ad on duplicate readings) is below 2.1 or above 2.45. It is concluded that the National Reference Scheme should be phased Out in favour of the Routine supply Scheme, and that even among most of the hosPimls in the latter scheme there is scope for greater accuracy, which the experience Of the Correspondence: Dr L. Poller, National Reference Laboratory for Anticoagulant Control Reagents, Withington Hospital. Manchester M20 8LR.