Point-of-care (POCT) prothrombin time monitors: is a periodical control of their performance useful?

Point-of-care testing (POCT) prothrombin time monitors are now widely used to monitor oral anticoagulant treatment. Although portable coagulometers are extremely easy to use, checking the quality of their performance presents some difficulties.

The aims of this study were to investigate on a quarterly basis the performance of 95 Coagucheck S assigned to 99 anticoagulated patients at home. This was done checking the monitors versus a reference coagulometer in the laboratory at our Thrombosis Centre (TC). The other aims were to carry out an external quality assessment employing different sets of INR certified plasmas with 5 different ranges of anticoagulation and to assess the performance of the different lots of strips employed by the patients during the study.

No difference between the PT INR obtained with both the systems at the first quarterly check was noted but a significant difference was found when the two systems were compared at the second and third quarterly checks. The Bland-Altman test showed increased disagreement between the first and the third controls. The percentage of INR values that showed a difference of more or less than 0.5 INR units in the PT values performed with both the systems was: 1.0% (first control), 7.5% (second control) and 11.5% (third control) (Chi-Square: 8.315, p=0.0156). Lots with differences higher than 10% in terms of +/- 0.5 INR Units at the first, second and third controls were 16%, 20.8% and 61%, respectively. Seven monitors (7.3%) failed to test one or two of the INR certified plasmas of one set but performed well using a second set of plasmas. Three monitors (3.1%) failed to test two sets of plasmas but performed well using a different lot of strips (from 279A to 483A). One monitor (1%) gave unsatisfactory results with different sets of plasmas and strips. All the other PT INR obtained with the monitors fell well within the different ranges of the INR certified plasmas.

Anticoagulated patient in self-testing or self-management should periodically bring their portable coagulometer to a reference Thrombosis Centre especially when the lot of strips have to be changed. The role of Thrombosis Centre appears therefore crucial in this regard.