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Letters |
1
Dept. of Pathol., Univ. of Mississippi Med. Center, 2500 N. State St., Jackson, MS 39216,
2
Dept. of Pathol., Hennepin County Medical Center, 701 Park Ave., Minneapolis, MN 55415.
a Author correspondence.
To the Editor:
Cardiac troponin I (cTnI) is both highly sensitive and specific for the detection of myocardial injury (1)(2), and cTnI now challenges creatine kinase MB (CK-MB) as the optimal marker for acute myocardial infarction (3)(4). The Stratus II batch analyzer (Dade International, Miami, FL) uses monoclonal anti-cTnI antibodies immobilized on a glass fiber support for the measurement of serum or plasma cTnI. During the course of clinical evaluations and implementation of cTnI testing on the Stratus II, we noted occasional falsely high results on serum specimens that were found to be either within the reference interval or below the detection limit upon reanalysis. It appeared that incomplete clotting of the specimen contributed to false-positive cTnI values. This letter describes our efforts to eliminate these analytical false-positive cTnI results.
Results for cTnI on 20 serum specimens (2.2%) from >900 patient specimens tested over 3 months were challenged by clinicians as false positives, with values ranging from 1.9 to 18.8 µg/L. Some but not all of the patients from whom these samples were collected either had a coagulopathy or had received an anticoagulant. The Stratus package insert for cTnI indicates that serum and plasma are acceptable specimens and that care should be taken with patients on anticoagulant therapy. The package insert for CK-MB on the Stratus indicates that serum is the only acceptable specimen. Often, both CK-MB and cTnI tests are performed on a stat basis. Three approaches were tested to eliminate analytical false-positive cTnI results on serum specimens. First, serum was poured into heparin tubes before analysis. Comparison of plasma and serum specimens drawn in parallel showed good agreement (r = 0.99, slope = 0.91 ± 0.02, intercept = 0.09 ± 0.07, Sy|x = 0.22) in 21 samples ranging from 0.2 to 6.4 µg/L [in practice at Hennepin County Medical Center (HCMC)]. Second, serum samples were centrifuged a second time for 10 min to remove any residual fibrin, as recommended by the manufacturer [in practice in combination with SuperSerum at University of Mississippi Medical Center (UMMC)]. Third, SuperSerum (ITC, Edison, NJ), which contains protamine sulfate, thrombin, and snake venom to enhance coagulation, was added to serum, incubated for 10 min, centrifuged for 10 min, and then analyzed for cTnI (in practice at UMMC). Comparison of serum before and after SuperSerum treatment showed good agreement (r = 0.99, slope = 0.92 ± 0.01, intercept = 0.19 ± 0.10, Sy|x = 0.34) in 16 samples ranging from 0.0 to 29 µg/L. When these approaches were evaluated in routine practice with ~1200 specimens, there were no physician challenges, and on retrospective review only two results (0.17%) were increased (2.0 and 2.7 µg/L) without other evidence of acute myocardial injury such as an abnormal electrocardiogram, increased CK-MB, or another specimen with an increased cTnI. Both of these patients had congestive heart failure and past histories of myocardial infarction.
The routine use of plasma samples in our institutions has essentially eliminated the interference we have described. To avoid false-positive cTnI results when analyzing serum on the Stratus II, any of the three approaches described above could prove useful. The following protocol has been implemented for serum samples at HCMC: (a) If the initial result on a patient is >0.9 µg/L, check for clots and rerun that sample after pouring it over into a heparin tube. (b) If the current result from a patient is >0.9 µg/L and the previous result was <0.8 µg/L, check for clots and rerun the current sample. For example, if the current result is 1.3 µg/L and the previous result was 0.8 µg/L, rerun the current sample. (c) If the current result is <0.8 µg/L and the previous result was >0.9 µg/L and is not part of a downward trend from previous higher results, check for clots and rerun the previous sample. For example, if the current result is <0.4 µg/L and the previous result was 2.1 µg/L, check for clots and rerun the previous sample. If the previous sample has already been rerun, it is not necessary to run it again. In all cases the repeat value is reported.
References
Dade International, Miami, FL \.
a Author correspondence.
To the Editor:
The issue raised in this letter is related to specimen integrity as determined by preanalytical specimen preparation. All automated assay techniques can be compromised at some point in the analytical process when initial specimen preparation is inadequate or sample integrity is compromised. The package insert instructions of analytical systems provide guidelines and discuss limitations. Dade International recommends that laboratories follow the NCCLS guidelines (H18-A) for sample preparation and notes that special care should be taken when samples are obtained from patients on thrombolytic therapy, where samples may not clot completely within the above suggested time interval.
The following articles in journals at HighWire Press have cited this article:
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  C. Roongsritong, I. Warraich, and C. Bradley Common Causes of Troponin Elevations in the Absence of Acute Myocardial Infarction: Incidence and Clinical Significance Chest, May 1, 2004; 125(5): 1877 - 1884. [Abstract] [Full Text] [PDF]
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 P. Beyne, J.-P. Vigier, P. Bourgoin, and M. Vidaud Comparison of Single and Repeat Centrifugation of Blood Specimens Collected in BD Evacuated Blood Collection Tubes Containing a Clot Activator for Cardiac Troponin I Assay on the ACCESS Analyzer Clin. Chem., November 1, 2000; 46(11): 1869 - 1870. [Full Text] [PDF]
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 A. S. Jaffe, J. Ravkilde, R. Roberts, U. Naslund, F. S. Apple, M. Galvani, and H. Katus It's Time for a Change to a Troponin Standard Circulation, September 12, 2000; 102(11): 1216 - 1220. [Full Text] [PDF]
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D. S. Ooi and A. A. House Cardiac troponin T in hemodialyzed patients Clin. Chem., July 1, 1998; 44(7): 1410 - 1416. [Abstract] [Full Text] [PDF]
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