COMPARISON OF THE RADIOMETER AQT 90 FLEX ANALYSER VERSUS BECKMAN COULTER DxI 800 FOR THE MEASUREMENT OF TROPONIN I.

L MACKAY, N CRINIS, Q LAM, R SCOTTBiochemistry Department, Austin Pathology, Austin Health, Heidelberg Vic. 3084, Australia

lisa.mackay@austin.org.au

INTRODUCTIONTroponin I (TnI) is the biochemical marker of choice for assessing patients presenting to the Emergency Department with cardiac symptoms. Given the urgent nature of such requests, a rapid turn-around-time (TAT) is an important prerequisite for troponin assays and one which makes point-of-care testing attractive to clinicians.We evaluated the performance of the TnI assay on the Radiometer AQT90 Flex analyser to determine its suitability as a point of care option for our tertiary hospital Emergency Department.

METHODEvaluation of the AQT90 Flex TnI was performed within the laboratory. Analytical precision was evaluated using a low positive patient serum pool and two levels of a commercially available quality control material (Bio-Rad).For the correlation study, 165 EDTA plasma patient samples with a corresponding serum/lithium heparin plasma TnI ≥ 0.01 ug/L on the Beckman Coulter AccuTnI (Unicel DxI 800) were selected. All EDTA samples were analysed within four hours of collection on the AQT90 Flex. Statistical analysis was performed using a commercial statistics program (Analyse-it, version 2.09)

RESULTS & DISCUSSIONFigure 1 highlights characteristics of both TnI assays. Although analysis time is lower with the AccuTnI assay, sample transportation to the laboratory and sample preparation adds to overall TAT of the AccuTnI result. CONCLUSION

The results generated by the AQT90 Flex TnI assay are numerically different to the AccuTnI DxI 800 assay but with the application of an appropriate slope and intercept, the AQT90 Flex TnI assay can be used in the Emergency Department setting to produce results comparable to the laboratory assay. However, it is important that clinicians are aware that the assays are fundamentally different.

Correlation by linear regression yielded the equation:

DxI = 4.394AQT – 0.105, (R2 = 0.90)

across a DxI range of 0.02ug/L to 57.84 ug/L. See Figure 2a.Using the respective 99th percentile cut-off for each assay, 121/165 paired samples agreed in interpretation (i.e. elevated or negative by both assays).44 did not agree (see Figure 2b). All 44 were above the DxI cut-off but not the AQT. It was beyond the scope of this evaluation to determine which assay performed better clinically. However, it is important to note that the assays are not traceable to a common calibrator source and that the respective cut-offs were determined on different reference populations and hence not directly comparable. Furthermore, Beckman Coulter has since re-formulated the AccuTnI to improve performance at the low end.

Figure 1. Assay characteristics of the AQT90 Flex TnI versus the AccuTnI as described by the manufacturers.

Figure 2 a. Overall correlation between the AQT90 Flex TnI versus the AccuTnI. b. Correlation at low TnI values with respective 99th % reference population cut-offs.

REFERENCES

1. AQT90 Flex. TnI Test Kit package Insert. Radiometer 2008.2. AccuTnI package insert Ref 33340. Beckman Coulter 2006.

ACKNOWLEDGEMENTSWe thank Radiometer for the supply of reagent and instrument for this study

The coefficient of variation (CV) for the patient pool (mean 0.043 ug/L) was 11.2%. Using QC material, CV was 9.1% at a level of 0.42 ug/L and 11.9% at a level of 1.64 ug/L.

Correlation using paired patient samples - low values

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Minimum volume 2ml 200uLAnalysis Time 18 min 13 min99th % ref pop. 0.023 ug/L 0.04 ug/L

Calibrator traceability

NIST SRM 2921 EN ISO 17511