1 Department of Anaesthesia and 2 Department of Clinical Perfusion, Leeds General Infirmary, Great George Street, Leeds LS1 3EX, UK
Corresponding author. E-mail: michael.cross@leedsth.nhs.uk This article is accompanied by Editorial I.
Accepted for publication: September 9, 2002
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Abstract |
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Methods. Blood samples were taken at the beginning of surgery and the PT and APTT was measured both in the laboratory and by the Hemochron® Response. The tests were repeated 30 min after reversal of heparin with protamine.
Results. Before bypass, the bias for PT was only +0.34, with small 95% limits of agreement. Making the same measurements after bypass, the Hemochron® Response under-read and the bias was 3.27, with an increase of the 95% limits of agreement. With the APTT, the bias and the 95% limits of agreement were greater before bypass, and became even wider after bypass.
Conclusions. We found good agreement in the PT and clinically acceptable levels of agreement in the APTT during the pre-bypass period. After bypass, bias became greater for both PT and APTT and the limits of agreement could be clinically unacceptable.
Br J Anaesth 2003; 90: 499501
Keywords: complications, coagulopathy; heart, cardiopulmonary bypass; measurement techniques, coagulation; surgery, cardiovascular
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Introduction |
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We compared the results from a new NPT system (Hemochron® Response Whole Blood Coagulation System, International Technidyne Corporation Limited, Margate, Kent, UK) with laboratory results in samples from patients undergoing cardiac surgery using cardiopulmonary bypass. The equipment used to measure PT and APTT is also used to measure the ACT during the intraoperative period, and is therefore widely available in many cardiac operating theatres.
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Methods and results |
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The samples were taken by a single observer to standardize the technique and minimize sampling errors. After aspirating 8 ml from an arterial cannula (to clear the dead space), another 8 ml of blood was aspirated for the study. Exactly 2 ml of blood was transferred into each of the PT and APTT assay tubes (as described by the manufacturer). The tubes were vigorously shaken to achieve even mixing of blood and activation of the reagent before placing in the machine. Simultaneously, 1.4 ml of citrated blood was sent to the laboratory as a control. The tests were performed at two times; before systemic anticoagulation with heparin; and 30 min after reversal of heparin with protamine.
The Hemochron® Response measures PT, APTT, ACT and also estimates the international normalized ratio. Glass tubes containing a specific activator for PT and APTT and a small magnet are partially filled with blood and placed in the machine. The activating agent used in the PT tube is rabbit brain thromboplastin and in the APTT tube is kaolin and platelet factor substitute. Two magnetic detectors located in the test well continuously monitor the precise position of the magnet. When a specific displacement of the magnet occurs, the time between the beginning of the test and this clot end-point is displayed as the coagulation time for fresh whole blood. The plasma PT and APTT are subsequently calculated from the whole blood clotting value and displayed immediately.
In the laboratory, PT was measured using Instrumentation Laboratory (IL) PT-fibrinogen recombinant reagent and the ACL thousand series coagulation analyser. PT-fibrinogen recombinant reagent is a mixture of recombinant rabbit tissue thromboplastin, synthetic phospholipid and calcium ions. The ACL analyser determines the coagulation time by monitoring turbidity during coagulation. IL APTT reagent is a mixture of micronized silica and bovine brain cephalin. A mixture of plasma and APTT reagent is incubated for 5 min before being recalcified with a volume of calcium chloride. The coagulation time is determined by the ACL coagulometer in the same way as the PT using a combination of centrifugal and nephelometric analysis. Bias analysis, according to Bland and Altman,2 was used to test the agreement between the results.
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Comment |
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The Hemochron® Response gives APTT and PT results rapidly following cardiac surgery but the accuracy and precision of the results means that the monitor is unsuitable for assessment of coagulopathy following cardiac surgery.
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References |
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