Blood Collection Tubes vs Thromboplastin Reagents for INR
Blood Collection Tubes vs Thromboplastin Reagents for INR
International normalized ratio (INR) discrepancies were noted between clinical laboratories using various prothrombin time (PT) systems. We studied the influence of different commercial blood collection tubes and different PT systems on INR measurements. INRs of fresh patient samples were determined by 3 laboratories, each using different PT systems. In the first part of the study, samples were drawn with Vacutainer tubes and in the second part with Monovette tubes. In the first part of the study, the maximum bias for all patients amounted to 0.46 INR (14%), and in the second part, to 0.14 INR (4.9%). The maximum bias for all patients could be reduced further by local system calibration using frozen pooled plasma specimens. The sodium citrate solutions in the blood collection tubes were contaminated with magnesium ions (approximately 2.7 mmol/L and 0.3 mmol/L in the Vacutainer and Monovette, respectively). INR discrepancies could be explained largely by this influence of blood collection tubes. The maximum allowable magnesium contamination in sodium citrate anticoagulant solutions should be less than 1 mmol/L.
All medical staff and health auxiliaries involved in controlling oral anticoagulant treatment with vitamin K antagonists should be encouraged to use the international normalized ratio (INR) system. This is especially important in the case of any treated patient who is likely to be transferred to another laboratory where a different modification of the prothrombin time (PT) test is likely to be used. The Netherlands has developed a nationwide system of regionally centralized anticoagulant control units for outpatients. Each individual Dutch center is called a Thrombosis Service. Control of in-hospital patients is excluded from the duties of a Dutch Thrombosis Service. When an outpatient controlled by a Thrombosis Service must undergo a medical procedure in a hospital, the anticoagulant control is deputed to the hospital's laboratory. We observed an INR discrepancy between a Thrombosis Service and a neighboring hospital laboratory, each using a different reagent-instrument combination. The INR measured by the hospital laboratory was systematically lower than the value provided by the Thrombosis Service. The discrepancy gave rise to postponement of elective cardioversion for a number of patients with atrial fibrillation because the hospital preferred the INR to be higher than 2.5 in the period closest to cardioversion. Approximately 3% of planned cardioversions were cancelled because of INR discrepancies. The purpose of the current study was to find the origin of the discrepancies and to suggest potential corrective measures.
Abstract and Introduction
Abstract
International normalized ratio (INR) discrepancies were noted between clinical laboratories using various prothrombin time (PT) systems. We studied the influence of different commercial blood collection tubes and different PT systems on INR measurements. INRs of fresh patient samples were determined by 3 laboratories, each using different PT systems. In the first part of the study, samples were drawn with Vacutainer tubes and in the second part with Monovette tubes. In the first part of the study, the maximum bias for all patients amounted to 0.46 INR (14%), and in the second part, to 0.14 INR (4.9%). The maximum bias for all patients could be reduced further by local system calibration using frozen pooled plasma specimens. The sodium citrate solutions in the blood collection tubes were contaminated with magnesium ions (approximately 2.7 mmol/L and 0.3 mmol/L in the Vacutainer and Monovette, respectively). INR discrepancies could be explained largely by this influence of blood collection tubes. The maximum allowable magnesium contamination in sodium citrate anticoagulant solutions should be less than 1 mmol/L.
Introduction
All medical staff and health auxiliaries involved in controlling oral anticoagulant treatment with vitamin K antagonists should be encouraged to use the international normalized ratio (INR) system. This is especially important in the case of any treated patient who is likely to be transferred to another laboratory where a different modification of the prothrombin time (PT) test is likely to be used. The Netherlands has developed a nationwide system of regionally centralized anticoagulant control units for outpatients. Each individual Dutch center is called a Thrombosis Service. Control of in-hospital patients is excluded from the duties of a Dutch Thrombosis Service. When an outpatient controlled by a Thrombosis Service must undergo a medical procedure in a hospital, the anticoagulant control is deputed to the hospital's laboratory. We observed an INR discrepancy between a Thrombosis Service and a neighboring hospital laboratory, each using a different reagent-instrument combination. The INR measured by the hospital laboratory was systematically lower than the value provided by the Thrombosis Service. The discrepancy gave rise to postponement of elective cardioversion for a number of patients with atrial fibrillation because the hospital preferred the INR to be higher than 2.5 in the period closest to cardioversion. Approximately 3% of planned cardioversions were cancelled because of INR discrepancies. The purpose of the current study was to find the origin of the discrepancies and to suggest potential corrective measures.
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