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Letters to the Editor |
Department of Pathology, and Laboratory Medicine, Hospital of the, University of Pennsylvania, Philadelphia, PA
aAddress correspondence to this author at: Hospital of the University of Pennsylvania, Department of Pathology and Laboratory Medicine, 3400 Spruce St., Philadelphia, PA 19104-4283. Fax 215-662-7529; e-mail kricka{at}mail.med.upenn.edu.
To the Editor:
Central tenets of quality control (QC) and quality assurance are that (a) controls should be handled by the regular staff, and (b) they should be treated the same way as an equivalent patient specimen. This is not always adhered to in routine practice, and recent problems with blood collection tubes underline this point (1)(2)(3)(4). Collection of blood in certain batches of Vacutainer® SST tubes was associated with errors in some immunoassays on certain immunoassay platforms [e.g., biases of 20% to 30% for total thyroxine (T4), total triiodothyronine (T3), and cortisol and –15% to 20% for follicle-stimulating hormone (FSH)]. These errors have been attributed to contamination of samples with surfactants that are used to coat the tubes; the surfactant can release reagent antibodies from some solid-phase materials used in immunoassay methods (5)(6).
Blood specimens for testing are removed through a metal needle into a blood collection tube. The tube and its contents are centrifuged to isolate serum, and the serum is either sampled directly from this tube or transferred to another tube from which a sample is removed for analysis. In contrast, a QC material is poured into a sample cup or tube from which it is sampled for analysis. Thus, the control material, unlike the blood specimen, does not come into contact with the surface of a metal needle or the contents of a blood collection tube.
We reasoned that adverse effects of additives in blood collection tubes would be apparent if control materials were poured into blood collection tubes and processed in the same way as blood. To test this, we analyzed control materials that were poured into our current batch of SST tubes or into SST tubes from the variant lot shown to be the source of an immunoassay interference. We did not introduce the control material through a needle because we, like other laboratories, strive to limit the use of needles. The tubes were then processed as if they were routine blood specimens, and the materials were analyzed.
We used 3 control materials: Performance Verifier 1 (lot no. L5782; Ortho Clinical Diagnostics), Lyphochek Immunoassay Plus Control (lot no. 40160), and Lyphochek Anemia Control (lot no. 43100; both from Bio-Rad Laboratories). We also used 3 types of blood collection tubes: BD Vacutainer glass tubes and BD Vacutainer plastic SST tubes (Becton Dickinson) from our routine supply, and BD Vacutainer plastic SST tubes from the variant lot of tubes (supplied by Becton Dickinson). Analytical results obtained for the chemistry and endocrine tests were compared with those obtained by analysis of the control material directly from the bottle.
For common chemistry tests (glucose, urea, creatinine, sodium, potassium, chloride, iron, carbon dioxide, amylase, lipase, calcium, magnesium, phosphate, cholesterol, triglycerides, uric acid, total protein, albumin, aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, creatine kinase, 
-glutamyl transpeptidase, alkaline phosphatase, total bilirubin, conjugated bilirubin, and unconjugated bilirubin) performed on the Vitros 950AT (Ortho Clinical Diagnostics), we observed no differences between results for the control analyzed directly or after processing in the current or in the variant lot of SST tubes. Results for all replicates of the controls were within the established QC ranges for all analytes for all of the samples. This is in accord with the absence of any reported biases for common chemistry tests in tubes from affected batches.
By contrast, the endocrine tests run on an Immulite 2000 (Diagnostic Products Corporation) showed significant processing-dependent differences in the results for the analytes previously affected by surfactants in SST tubes (Table 1
). Total T4 results for all of the controls processed in SST tubes from the variant lot were above the established QC intervals in all tubes (triplicate) and at all concentrations of the controls. Several of the total T3 and cortisol results for the QC samples were at or above the established QC interval and would be regarded with suspicion (3 of 9 tubes for T3 and 2 of 9 tubes for cortisol). The results for FSH had the reported negative bias, and a substantial number of QC results (5 of 9 tubes) were at or outside the limits of the established QC intervals. As expected, for the controls processed in the current lot of SST tubes and for controls that had not come into contact with an SST tube, control limits were not exceeded for the overwhelming majority of results (70 of 71 results, with 1 missing value).
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Other factors, such as the volume of the control material, length of incubation (
2 h) in the tube, and contact with the stopper and its lubricant, may influence the magnitude of tube effects. We filled tubes with only 1 mL of control material so as to produce high concentrations of additive and thus maximize any possible effects of the additives on the assays investigated in this study.
Our results emphasize the benefit of strict adherence to one of the basic tenets of QC, namely, that control specimens should be treated identically to specimens from patients. This strategy has revealed the interference attributable to the additives in the variant lot of SST tubes and, had it been in place, would have alerted the laboratory to the interference. We recommend that laboratories consider including this strategy in their QC plan. Periodic or routine processing of controls in blood collection tubes should provide a timely warning of possible interferences by additives in blood collection tubes.
References
The following articles in journals at HighWire Press have cited this article:
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  A. K. Stankovic and G. Parmar Assay interferences from blood collection tubes: a cautionary note. Clin. Chem., August 1, 2006; 52(8): 1627 - 1628. [Full Text] [PDF]
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