Filter Paper-collected Blood Lead Testing in Children

The Editorial “Filter Paper Lead Testing” by Moyer et al. (1) repeats previously raised hypothetical and unsubstantiated claims on filter paper (FP collection-based) blood lead testing (1)(2)(3)(4). All the theoretical charges were appropriately rebutted in the past based on published evidence (5)(6)(7)(8)(9). The peer-reviewed scientific literature supports the clinical usefulness of FP-collected blood samples in childhood lead poisoning screenings (10)(11)(12)(13)(14)(15)(16)(17)(18)(19). The Editorial’s authors (1) used references that do not support their statements: one of the references lacks entirely the topic of lead testing for which it was used. Worse yet, available references were omitted from the Editorial that support FP collection-based methods. The following comments and references attempt to correct these mistakes and provide accurate documentation from the scientific literature on FP. Reference 9 in the Editorial (14) is cited for support of “low extraction recoveries” of lead from FP. This study is based on a method that is a direct analysis that does not require an extraction (14). The same reference is cited to support the statements that contamination occurs as a result of “high and variable lead content of the FP or by contamination encountered in the sampling process”. The reference did not address FP blank lead concentration issues or that contamination caused any unusual problems during sample collection. The wrong volume and page numbers are cited for reference 8 in the Editorial (18). The work of Cernik (18) and O’Broin et al. (20) are used to support “nonreproducibility of blood spotting because of differences in hematocrit [Hct] and hemoglobin [Hb]” (1). Cernik (18) studied neither Hct nor Hb. His article investigated the effects of paper punch size on reproducibility of blood lead measurement on FP. He reported that punches <4.0 mm in diameter caused variation in reproducibility but that punches >4.0 mm correlated excellently with volumetric measurements. Laboratories in the 1990s use 6.0 mm (1/4-inch) or 4.5 mm (3/16-inch) punch sizes with excellent reproducibility (11)(12)(13)(14)(15)(16)(17). Similarly, the study by O’Broin et al. (20) (reference 11 in the Editorial) did not examine blood lead measurement on FP. Their conclusion was that: “Specific studies on the distribution in a paper matrix of all analytes of interest are desirable”. The cited study (20) is applicable only to ferritin, cobalamins, and folates. Lead was not an analyte in their study. However, an earlier study by O’Broin (21) has some relevance to FP measurement of heavy metals that distribute mainly into erythrocytes. Lead is 94% distributed into erythrocytes (22). Two analytes were examined from FP blood samples for accuracy at a wide range of Hct values (19–63%). 125I-labeled thyroxine was seriously influenced, but chromium-51 was hardly affected by the large Hct variation. O’Broin’s conclusion was that analytes that distribute into plasma are affected by the wide variation of Hcts but analytes in erythrocytes are not significantly influenced (21). O’Broin’s findings (21) were specifically confirmed for blood lead measurement on FP in recent studies that measured Hb or Hct along with FP and liquid blood lead concentrations (13)(17). Schonfeld et al. (13) in a study of >1500 children reported an average Hb of 120 g/L [12 g/dL Hb (range, 8.3–18.7 g/dL)]. They stated, “Both fingerstick lead screening tests had excellent correlation with the confirmatory VPb [venous lead]”. The authors concluded that “screening for lead poisoning in urban areas is feasible by direct measurement of fingerstick CPb [capillary/Microtainer lead] or FPb [FP lead]”. Yee and Holtrop (17) specifically studied the Hct effects on blood lead quantification. Fingerstick FP blood and simultaneous venous specimens were collected (n = 59) and analyzed by graphite furnace atomic absorption spectrometry. Hct values ranged between 27% and 44%. The variation did not affect the excellent correlation between FP and venous blood lead results. The Editorial (1) implies that one cannot achieve accurate results using FP collected samples because of spotting difficulties. On the precision of FP blood lead measurement, Morrow et al. (23) of the CDC stated, “Filter paper had excellent correlation to target values (r = .998) with precision similar to that of liquid samples”. The Editorial (1) states that “Bias may be introduced by high and variable lead content of the FP …”. In 1992, Wang et al. (12) reported, “A random sample of 30 blank filter paper spots from our neonatal sampling sheet was tested for lead. No detectable levels of lead were found in any of the blank filter paper spots tested”. Morrow et al. (23) stated, “lead in the filter paper was generally low (<2 mcg/dL) during the last 5 years (1988–1993). Therefore these lots appear to be suitable for measuring low levels of lead”. Yee and Holtrop (17) in 1997 tested Schleicher and Schuell 903 FP (S&S 903). They reported that “filter paper blank values were <2 mcg/dL [20 μg/L] with the use of aqueous standards without correction and 0.1 mcg/dL with a paper blank correction; and during the collection of 163 samples from 6 different urban sites over a period of 1 year no environmental lead contamination was encountered, based upon the good agreement between all of the paired FP and venous lead results”. Verebey (6) stated, “in Leadtech as part of the QA and QC program all new lots of filter paper (Schleicher & Schuell 903 filter paper) are tested for background lead. Of the 156 lots examined (n = 780 tests) in the past 4 years, not one blank filter paper was found with blood lead equivalent ≥1 μg/dL (10 μg/L).” Wong et al. (24) (reference 6 in the Editorial) did report high blank FP lead values in an abstract, which was never published in a peer-reviewed scientific communication. Their methodological problem may have led to the very high background blank filter paper results. Yet this was the sole study chosen by the authors of the Editorial to support their statement. Contrary to the findings of Wong et al. (24), four different laboratories found the blank FP (S&S 903) values low and acceptable for blood lead screening procedures (6)(12)(17)(23). The Editorial (1) suggests that contamination of blank FP collectors caused imprecision during the first two cycles (trial period) in the Wisconsin State pilot proficiency testing (PT) program (19). The suggestion is not feasible because all laboratories participating successfully used the same (S&S 903) filter papers routinely in their practice and in the PT program. The initial performance variations were likely attributable to calibration and methodological adjustments to the new PT program. Reading the Editorial (1), one gets the notion that FP has not been successfully used in large clinical studies and that, therefore, we have no idea how this “new procedure” would perform in the “field”. Thus, the authors of the Editorial suggest that FP collection is uniquely predisposed to “preanalytical contamination” and analytical variations. The preanalytical contamination and analytical concerns are ones that any competent analyst must deal with in the performance of trace metal analysis. There is no need to insinuate that these concerns are unique or apply only to FP lead testing. In two of the worst examples with Microtainer capillary collectors, as many as 68% and 70% of positive results were false positives attributed to preanalytical contamination (25)(26). In Texas, Medicaid released a provider alert (Bulletin No. 115) indicating that a batch of blood collection tubes used for fingerstick collection were contaminated with the equivalent of as much as 250 μg/L lead. It is clear that contamination theoretically can cause preanalytical problems with FP collection as well as other capillary blood collections, but it was not noted as a unique problem in several major studies that used FP in the 1990s. Another criticism in the Editorial (1) claimed that the good performance of FP methods in the Wisconsin State Pilot PT program (19) was attributable to the controlled “artificial environment” and that it would not be expected to perform similarly well in a day-to-day practice. This statement is purely hypothetical. FP has been used successfully in several large studies in clinical settings in the United States and other countries without any unusual analytical or contamination problems (10)(12)(13)(14)(15)(16)(17)(27)(28): Schonfeld et al. (13) studied >1500 children in Connecticut; Schlenker et al. (27) studied 5350 children in Salt Lake City County; Pfitzner et al. (28) studied 218 children in Nigeria; and Verebey (5) studied >94 964 children across the United States. The average blood lead concentrations of children in the Verebey study (5) was 30 μg/L (3 μg/dL), closely matching the CDC’s NHANES national average (29). Piomelli et al. (10) studied 106 people in the Himalayas, using FP-collected blood. The average blood lead in the Himalayas study was also 30 μg/L (3 μg/dL). These low concentrations (5)(10) could not be measured if the FP itself had “high and variable lead content”, as suggested by the Editorial (1) and Wong et al. (24). The clinical significance of screening tests is to identify subjects with blood lead concentrations >100 μg/L (10 μg/dL) and to avoid false-negative tests. Yee and Holtrop (17) compared 119 field samples in which venous blood and fingerstick FP collections were made simultaneously. At the 100 μg/L (10 μg/dL) cutoff, the false-positive and false-negative rates were 0.8% and 1.7%, respectively. The correlation was r = 0.975, with the regression equation: y (FP) = 1.37 + 0.83x (venous). Thus, the FP-based screening tests do fulfill the basic requirements needed to identify children with increased blood lead concentrations. The NHANES statistical data (29) indicate that in the United States, ∼95.7% of the screening test results are expected to be <100 μg/L (10 μg/dL). Each clinical laboratory screening report states that a higher lead result must be confirmed by a venous specimen. No clinical treatment decisions are made based on an increased screening result alone. The excellent precision shown by the FP screening test in numerous field studies (12)(14)(16)(17) and in the new Wisconsin State PT (19) progam is comparable in performance to confirmatory methods. Therefore, the FP-based methods are more than adequate to serve the screening functions they are intended to perform. The advantages of FP blood collection is well stated in the Editorial (1): “(a) a relatively small amount of blood is required; (b) blood collection via fingerstick is generally easier and less traumatic to children; (c) collection can be performed by a relatively untrained collector; and (d) appropriately prepared FP specimens are stable and can be easily transported to the laboratory”.