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Letters |
1
Wisconsin State Laboratory of Hygiene, Toxicology Section, Madison, WI 53707
2
Centers for Disease Control, and Prevention, National Center for, Environmental Health, Atlanta, GA 30341
a Author for correspondence. Fax 608-224-6259; e-mail nvstox{at}mail.slh.wisc.edu
To The Editor:
The recent Editorial on filter paper (FP) lead testing (1) provides a needed discussion of preanalytical variables in FP testing. These issues were not investigated in our report on FP proficiency testing results (2) that provided the basis for the Editorial. We would like to comment on two statements made in the Editorial.
First, the authors assert that improvements in participant performance that were observed following an initial testing period are attributable to a change in the source of FP sheets during the study (1). We believe a close examination of the data does not support this hypothesis. To review, our study (2) reported FP data from the period October 1997 through June 1998. FP specimens were grouped in monthly testing events, each consisting of five specimens. Events in October and November 1997 were considered preliminary, and the data from these events showed wide scatter. Beginning with the December 1997 testing event, the FP results were evaluated using CLIA acceptability criteria for blood lead. We reported that a substantial improvement in accuracy coincided with the institution of this formal evaluation and continued throughout the rest of the study period. We attributed this improvement to changes made by the participating laboratories during or immediately following the preliminary events (2).
As the Editorial correctly notes, beginning with the January 1998 test event, the FP sheets used were from a single lot obtained by the Wisconsin State Laboratory of Hygiene for study purposes. Before this, the FP sheets had been provided by the individual participating laboratories. The Editorial then suggests that the improvement in performance reflected the change in FP source (1). However, this hypothesis does not reconcile data from the December 1997 testing event (the first formally evaluated event), which were provided on FP sheets supplied by the participants, that showed the improved performance consistent with the later period (2). All of the FP sheets used in the study "shared the common characteristics of Schleicher & Schuell no. 903 composition, designated spotting and handling areas, and prior suitability testing by participants" (2). These shared characteristics are also contrary to the hypothesis stated in the Editorial. In addition, we received reports from some participants that they had made changes to their procedures before the initial evaluated test events. Because the details of these changes were not made known to us, they were not discussed. Upon consideration of these factors, we believe our original statement that the improvement in data quality can be attributed to changes made by participants is correct. Although the possible effect of FP sheets on test result quality merits discussion, our data do not support the contention that this was a factor in our results.
Second, the authors state that "Readers should not conclude ... that FP-Pb testing is a suitable replacement for venous blood lead testing" (1). We agree with this statement. It is well-established that regardless of the collection medium, capillary blood is suitable for screening purposes, but venous blood is required for repeat confirmation and medical management (3)(4). Our conclusions were made with the assumption that readers are aware of this fact.
References
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