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Troponin I at Mount Zion
|Performed by||Mt. Zion Chemistry|
|In House Availability||Test available 24 hours per day 7 days per week|
|Method||iStat two-site Enzyme linked immunosorbent assay|
|Collection Instructions||Due to the sample stability limits of this assay (30 min), samples should be delivered directly to the testing section of the Mt Zion clinical laboratory within 10 minutes of collection.|
|Container type||Light green top or Dark green top filled to full extent of vacuum|
|Amount to Collect||Full Light green or Dark green vacutainer tube|
|Sample type||Heparinized whole blood|
|Preferred volume||Full Light green or Dark green vacutainer tube|
|Min. Volume||Full Light green or Dark green vacutainer tube|
|UCSF Rejection Criteria||Clotted sample. Sample > 30 minutes old when received. Vacutainer not completely filled|
|Processing notes||Deliver immediately to lab section for testing|
|Normal range||< 0.09 µg/L|
|Critical value||> 0.08 µg/L|
|Turn around times||1 hour|
|Additional information||WARNING: Results for the iSTAT troponin test performed at Mt Zion cannot be directly compared to results for troponin tests performed in the central laboratory at Parnassus because of assay differences in standardization, normal cutoffs, and absolute values.
The Joint European Society of Cardiology/American College of Cardiology Committee (ESC/ACC) consensus guidelines for diagnosis of MI include observation of the typical rise and fall of cardiac markers with at least one of: ischemic symptoms, development of pathologic Q waves on electrocardiogram (ECG), ST segment elevation or depression in ECG, or coronary artery intervention (1); troponin is the preferred marker with a cutoff set at or above the 99th percentile of a healthy population. The troponin I assay used at Mt. Zion is manufactured by Abbott Laboratories (iSTAT troponin assay) and has a 99th percentile cutoff in normals of < 0.09 micrograms/L. All troponin I results that exceed 0.08 µg/L in this assay are flagged as abnormal. The coefficient of variation of this assay at the 99th percentile is 15 -25% as determined by the manufacturer and in house testing (Pathology 42:402-408, 2010). The coefficient of variation at a level around 0.15 to 0.2 micrograms/L is approximately 10%. This assay is not considered a high sensitivity troponin assay.
Serial sampling is strongly recommended to help guide interpretation of troponin results and detect the temporal rise and fall of troponin levels characteristic of acute cardiac injury. When following patients with troponin levels of < 0.25 in either the iSTAT or Parnassus central lab assays, one can be confident (95%) that a change in results of ~0.05 units or more is clinically real and not due to inherent variability in the assay. Changes less than ~ 0.05 units could represent inherent assay variability or be clinically real. When following patients with troponin levels of 0.25 or more, one can be confident that a change in results of 20-30% or more is clinically real and not likely due to assay variability. In patients with troponin levels of 0.25 or greater, a change in results of less than 20-30% could represent inherent assay variability or be clinically real.
The iSTAT point of care assay used at Mount Zion is not as sensitive as the Parnassus central lab troponin assay for detecting small elevations in troponin above the respective 99%ile normal cutoffs of each assay.
The iSTAT assay tends to miss borderline, low level troponin elevations based on the 99%ile cutoffs. A negative iSTAT result in a sample with a positive central lab troponin result at Parnassus is primarily observed in cases where the Parnassus central lab troponin value is borderline/slightly elevated (in the range of 0.05 – 0.10).
The iSTAT assay will be positive in 90% of cases where the troponin is > 0.10 in the Parnassus central lab assay. A sample positive by iSTAT will almost always (97-99%) be positive by the Parnassus central lab assay. In the occasional sample where an iSTAT result is positive and the Parnassus central lab result is negative, the iSTAT result is usually only borderline elevated. It is unknown whether these cases represent false positive results by iSTAT or false negative results by the Parnassus central lab assay.
Note: Troponin I values above the 99th percentile normal limit can occur due to many different causes of cardiac injury, not just acute coronary syndromes. In addition, > 90% of patients who have undergone radio-frequency ablation of an arrhythmogenic focus may exhibit an increase in troponin levels. In contrast to CK-MB which can be increased by skeletal trauma including the transection of muscle tissue during surgery, release of troponin I from non-cardiac sources has not been reported.
Spurious increases in troponin I can occur in samples that contain microclots. Collection of a heparinized blood specimen is recommended to minimize the chance of microclot formation. Heterophile antibodies or abnormal immunoglobulins may cause falsely increased or falsely decreased results; falsely low results may occur in patients with autoantibodies against cardiac troponins. Anti-E.coli antibodies may also cause anomalous results. Technical artifacts should be suspected in patients in whom an increased troponin level abruptly falls to normal much more quickly than would be expected, or in whom serial troponin levels are chronically elevated. If a spurious result is suspected, the laboratory can be notified to repeat the result using the same assay and/or a different troponin assay for confirmatory purposes.
|Last Updated||8/27/2012 12:01:46 PM|