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Letters to the Editor |
1 Servei de Bioquímica Clínica, IDIBELL–Hospital Universitari de Bellvitge, L’Hospitalet de Llobregat, Catalonia, Spain
aAddress correspondence to this author at: Servei de Bioquímica Clínica, Hospital Universitari de Bellvitge, Catalonia, Spain. Fax 33-93-260-75-46, e-mail xfa{at}csub.scs.es.
To the Editor:
Generally, clinicians and laboratorians consider that premetrologic variation is negligible if all premetrologic sources of variation are standardized (1), but it has been demonstrated that in many instances this is not so (2)(3)(4). The aim of this study is to quantify the premetrologic variation associated with 33 biochemical quantities frequently measured in clinical laboratories.
All premetrologic procedures (sampling, transport, centrifugation, handling, and distribution of the samples) were done according to the working instructions of our laboratory. We recruited 38 healthy volunteers. Each individual had two immediately consecutive withdrawals, one from each arm at random, done by two different phlebotomists; the pair of phlebotomists was different for each individual. The samples were obtained from the antecubital vein, using evacuated tubes without additives but with a gel barrier as serum separator (Vacutainer®; product no. 368510; Becton Dickinson). After the sampling process, the tubes were sent to the premetrologic area of our laboratory, where the samples were held for a time before being centrifuged. The tubes of blood obtained by the first phlebotomist were centrifuged under standardized conditions (1200g for 10 min at 25 °C) 30 min after the withdrawal, whereas the tubes obtained by the second phlebotomist were centrifuged under the same conditions as the others but 90 min after the withdrawal. After centrifugation, the tubes were distributed to the appropriate analyzers. A pair of samples from each individual was measured, and one of each pair, randomly chosen, was measured again.
The quantities measured and the corresponding analyzers were as follows: serum concentrations of apolipoprotein A1, apolipoproteins B, bilirubin (nonesterified), chloride, cholinesterase, HDL-cholesterol, L-lactate dehydrogenase, lipoprotein(a), pancreatic 
-amylase, peptidyl dipeptidase A, transferrin, and triacylglycerol lipase were measured with the Modular System (Roche Diagnostics); serum magnesium concentrations were measured with the Dimension RxL (Dade Behring); serum concentrations of CA 15-3 antigen, CA 19-9 antigen, CA 125 antigen, -fetoprotein, ß2-microglobulin, prostate-specific antigen, and rheumatoid factors were measured with the Elecsys 2010 (Roche Diagnostics); serum concentrations of C-peptide, cortisol, estradiol-17ß, follicle-stimulating hormone, insulin, luteinizing hormone, parathyroid hormone, progesterone, prolactin, somatotropin, testosterone, and thyroglobulin were measured with the Immulite 2000 (DPC); and serum osmolality was measured with the Krioskop 800cl (Slamed).
For each quantity, the general metrologic variance was calculated as:
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For each quantity, the metrologic within-run variance was calculated as:
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Finally, the premetrologic variance was obtained by subtracting sMw2 from sG2, and the corresponding CV was calculated using the mean of all measurements done for each quantity.
The CVs corresponding to premetrologic variation different from zero are shown in Table 1
; CVs corresponding to the day-to-day imprecision at physiologic concentrations observed during the study are also included for comparative purposes. According to these results, one third of the serum components included in the study had premetrologic CVs greater than our day-to-day imprecision (values in bold italics in Table 1
), and 37% had premetrologic CVs not negligible but lower than our day-to-day imprecision. The premetrologic CVs were negligible for the remaining 30% (apolipoproteins B, CA 19-9 antigen, luteinizing hormone, magnesium, pancreatic -amylase, prostate-specific antigen, rheumatoid factors, testosterone, triacylglycerol lipase, and serum osmolality). We think, however, that when the waiting time for samples before centrifugation is extended to 3 or 4 h, as sometimes occurs in a routine setting, the premetrologic CVs will be higher than the observed ones.
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The results in this study, as in two previous studies (3)(4), indicate that premetrologic variation should generally not be ignored. The errors in the premetrologic phase are originally systematic; on a day-to-day basis, however, these errors could be considered together as random phenomena, and the corresponding variance should be taken into account when estimating the uncertainty of measurement associated with patients’ results (5).
References
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