To aid in the evaluation of patients with acute chest syndrome (ACS), many devices for bedside testing of cardiac markers, including troponin T, troponin I, CK-MB, and myoglobin, have been developed.1–6 By providing physicians with key information, these devices decrease turnaround time, improve the patient triage process, and can provide more efficient utilization of hospital resources.
While qualitative devices have been used with success for diagnosis and triage, quantitative values enable a clinician to use the full clinical potential of cardiac markers. In recent years, numerous studies have established the role of troponin T in risk-stratification and therapeutic and clinical decision-making in ACS patients. The GUSTO-IIa7 and FRISC8 studies have shown that quantitative troponin T concentrations are related to prognosis and cardiac risk assessment. Other out-comes-based studies9,10 have established specific troponin T concentrations that can be used to select patients most likely to benefit from a particular therapeutic strategy, such as treatment with low molecular weight heparin drugs,11 or GPIIb/IIIa inhibitor drugs.12 To take full advantage of this enhanced clinical utility, a quantitative determination of troponin T is necessary.
Secondly, quantitative values are necessary to fulfill the recommendation for serial testing. The new international guidelines recommend use of serial testing; the new definition of myocardial infarction (MI) requires a typical rise and gradual fall of troponin.13 American Association for Clinical Chemistry (AACC) leaders recommend serial measurements be obtained before a patient is ruled positive or negative for AMI.14 If myoglobin is being measured in early triage protocols, use of the 1 to 2 hour delta myoglobin concentration is recommended to reduce specificity problems associated with a single measurement of this nonspecific marker.14,15 Serial measurements can also help a physician to differentiate the mechanism of myocardial injury between troponin elevations due to a recent acute ischemic event, or to a chronic cause of myocardial damage such as sepsis, renal failure, or congestive heart failure.16
Finally, interpretation of qualitative test strips can be extremely subjective and can lead to confusion and misdiagnosis.1 To eliminate these problems and allow emergency department (ED) physicians rapid access to the full utility of troponin T markers, Roche Diagnostics has developed the Cardiac Reader (Roche Diagnostics Corporation, Indianapolis, IN). The Cardiac Reader provides quantitative measurements of troponin T and myoglobin that are standardized to laboratory-based assays.
The Cardiac Reader System: Technology and Features
The Cardiac Reader provides convenient one-step quantitative determinations of troponin T or myoglobin in whole blood, standardized to laboratory-based assays, in 12 minutes or less. The single-use immunochemical test strips used in the Cardiac Reader have been described previously.1,2 Briefly, 150 uL of venous whole blood are added to the test well. Plasma migrates along the strip due to capillary action, and the analyte combines with two specific monoclonal antibodies: one that is biotinylated and one that is conjugated to gold sol particles. When it reaches the read zone, the resulting sandwich complex is immobilized by streptavidin in a stripe along the read window, producing a reddish line with an intensity related to the concentration of the analyte in the blood. The specific antibodies used in this assay are identical to the ones used in the laboratory-based Elecsys troponin T and Tina-quant myoglobin assays, respectively.
The Cardiac Reader measuring unit contains a charge-coupled device (CCD) camera that quantifies the intensity of the signal line and control line on the immunochemical test strips via reflectance measurements. The reflectance measurements are then converted into analyte concentration using electronically stored lot-specific calibration curves. The process of standardization of the Cardiac Reader to the laboratory-based system and calibration curve generation has been described previously.1,2 A barcode reader can be attached for convenient entry of patient identification numbers, and patient results can be printed out or transferred to a laboratory-based information system, using an optional unidirectional interface.
The Cardiac Reader returns a quantitative troponin T result after a 12-minute test cycle, but a positive result becomes apparent in as little as 3 minutes because the Reader produces an audible alarm signal and an LED lights up as soon as the troponin T result exceeds 0.1 ng/mL. The troponin T quantitative measuring range is 0.1 to 2 ng/mL. However, due to the increasing recognition that even minor elevations of troponin T represent clinically significant myocardial damage,13,17 the Cardiac Reader will detect TnT concentrations between 0.05 and 0.1 and report them as low. Values < 0.05 ng/mL are reported as negative and concentrations > 2.0 ng/mL are reported as high. For cardiac M myoglobin determinations, the Cardiac Reader displays a quantitative result in the measuring range of 30 to 700 ng/mL after an 8-minute test cycle. The precision ranges from 8 to 16% for troponin T and 5 to 10% for myoglobin.
Evaluation and Use of the Cardiac Reader
Table 1 lists analytical results obtained from various published multicenter and single-site evaluations of the Cardiac Reader involving comparison to laboratory-based troponin T measurements. Because the same antibodies are used on the rapid assay strips as in the laboratory-based assay, and the calibration curves are standardized to the laboratory-based assay, a high degree of correlation is obtained, as evidenced by correlation coefficients and a slope very close to one. This is especially important with troponin T, as it allows application of the literature-validated cutoffs for risk-stratification and therapy selection, which are supported by clinical trials. While concentrations obtained with other point-of-care (POC) troponin assays may show some degree of correlation with laboratory-based results, the reported cutoffs are not equivalent, and may lead to substantial confusion.3,24,25 No published cutoffs are available for any other POC troponin assay. Because the concentrations obtained with the bedside device correlate with the laboratory-based assay, there is no need to rebaseline the patient upon admission to the hospital. Instead, clinicians can use the concentrations obtained in the ED, as well those obtained from the central laboratory following admission, to follow serial troponin trends in the patient.
Besides the analytical correlation with the laboratory-based assay, Cardiac Reader troponin T measurements show a high degree of clinical concordance as well, as shown in Table 2. Diagnostic sensitivity and specificity, relative to the laboratory-based troponin T assay, are excellent when using the ROC-curve derived 0.1 ng/mL cutoff. Importantly, the Cardiac Reader has the ability to detect smaller amounts of troponin release, below the 0.1 ng/mL cutoff corresponding to the World Health Organization AMI definition. As mentioned above, troponin T concentrations between 0.05 and 0.1 ng/mL will be detected, yielding a result of low. A more detailed comparison of the clinical utility of the Cardiac Reader to the laboratory-based troponin T is performed by constructing a 3×3 grid, taking these qualitative concentration ranges into consideration. Patients are classified into one of three categories based on troponin concentrations (< 0.05 ng/mL, 0.05 to 0.1 ng/mL, and ≥ 0.1 ng/mL). The myocardial damage and cardiac risk increase from one category to the next. As shown in Table 2, the Cardiac Reader and the laboratory-based assay show similar abilities to classify patients into these diagnostic categories. These studies demonstrate that the Cardiac Reader can provide clinically relevant and useful information, comparable to that obtained by the laboratory-based assay.
Point-of-care cardiac marker assays aid in the rapid diagnosis and triage of chest pain patients. The Cardiac Reader system provides convenient bedside quantitative determinations of troponin T and myoglobin in whole blood. Use of a quantitative system such as the Cardiac Reader enables serial troponin analysis, an essential component of AMI diagnosis under the new international guidelines. The Cardiac Reader uses the same antibodies and is standardized to the laboratory-based Elecsys troponin T assay, allowing application of troponin T results to riskstratification and therapy selection decisions. Analytical and clinical correlations between the Cardiac Reader and the laboratory-based assays are excellent. The Cardiac Reader is suited for use in emergency departments and smaller outlying clinics, and hospitals where centralized laboratory testing is unavailable. The Cardiac Reader system provides accurate, timely, and actionable information to clinicians needing an assessment of myocardial damage.
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