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Letters |
1st Laboratorio Analisi Chimico-Cliniche, Spedali Civili, 25123 Brescia, Italy
a Author for correspondence. Fax 39 030 3995369; e-mail panteghi{at}osp.unibs.it.
To the Editor:
Free deoxypyridinoline (fDPD) is increasingly used as a specific marker of bone resorption (1). To date, the concentration of fDPD in urine has been measured using cumbersome HPLC or manual microtiter-based ELISA procedures, which require ~4 h to perform (2). Recently, two rapid, fully automated chemiluminescent immunoassays were developed: by Chiron Diagnostics for the ACS:180® analyzer and by DPC® for the Immulite® analyzer. Both immunoassays use a competitive format involving the same monoclonal anti-fDPD antibody from Metra Biosystems (3)(4). If sufficiently reliable, these assays can accommodate increasing testing demands with both controlled operating costs and dramatically reduced turnaround time. This study assessed their analytical performance for the routine measurement of fDPD in our clinical laboratory. In particular, as suggested previously (5), goals for precision and accuracy were based on the biological variation of fDPD excretion in urine from healthy premenopausal women, and the results obtained during the evaluation were compared with these to assess acceptability (6).
All measurements on the two instruments were performed according
to the recommendations of the manufacturers by the same trained
technician. Linearity in the working ranges of the tests (ACS:180,
2–350 nmol/L; Immulite, 7–300 nmol/L) was good (r =
0.999 for the ACS:180; r = 0.998 for the Immulite). The
data for the imprecision study, which used two pooled human urines, are
summarized in Table 1
. Eighty-one urine samples (first morning void) with fDPD
concentrations ranging from 13 to 184 nmol/L were assayed, using the
Pyrilinks®-D (Metra Biosystems) method as the reference
(x) and the two automated procedures (y) in a
correlation study. The following results were obtained: ACS:180 =
0.96 (±0.03)x - 0.8 (±3.0); Sy|x =
12.9 nmol/L; r = 0.9552; and Immulite = 0.99
(±0.03)x + 9.8 (±2.8); Sy|x = 12.1
nmol/L; r = 0.9629.
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When biology is used to set analytical goals, desirable
imprecision (CV) is less than or equal to one-half of the average
within-subject biological variation (i.e., for urinary fDPD, total CV
6.7%), and desirable inaccuracy (bias) is less than or equal to
one-quarter of the group (within-subject plus between-subject)
biological variation (i.e., for urinary fDPD, average bias 5.5%)
(6). From our experimental results, we conclude that the
ACS:180 assay is probably accurate but is too imprecise for the
between-day evaluation and that the Immulite assay shows good precision
but also has a significant, constant positive bias. The final
considerations depend on medical needs (7). Because the bone
markers are useful adjuncts in monitoring patients and not in screening
for bone disorders, low imprecision (at least as good as the goal) is
required, whereas some degree of inaccuracy is probably less important
(8).
We acknowledge the expert technical assistance of Cristina Serena. We also thank Chiron Diagnostics (Cassina de' Pecchi, Milano, Italy) and Medical Systems (Genova, Italy) for the generous loan of instruments and reagents to carry out the study.
References
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