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1
Wadsworth Center, New York State Department of Health, PO Box 509, Albany, NY 12201-0509.
2
Division of General Pediatrics, North Shore University
Hospital, Great Neck, NY 11021.
3
Division of General Academic Pediatrics, Nemours
Children’s Clinic, Arnold Palmer Hospital for Children and Women,
Orlando, FL 32806.
4
Long Island Poison Control Center, Winthrop University
Hospital, 259 First St., Mineola, NY 11501.
5
Wisconsin State Laboratory of Hygiene, 2601 Agriculture
Dr., PO Box 7996, Madison, WI 53707-7996.
6
Department of Environmental and Community Medicine,
University of Medicine and Dentistry of New Jersey, 675 Hoes Ln.,
Piscataway, NJ 08854-5635.
a Author for correspondence. Fax 518-473-7586; e-mail
patrick.parsons{at}wadsworth.org.
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Methods: Aliquots from up to 72 blood lead performance pools from the New York State Department of Health and the Wisconsin State Laboratory of Hygiene were disguised as routine patient specimens and submitted in two phases to up to 42 certified clinical laboratories for blood lead analysis. These 42 laboratories also received aliquots of the same performance samples for blood lead analysis directly from the "open" PT program provider.
Results: Data reported under blind and open strategies were scored against acceptable target ranges using the Clinical Laboratory Improvement Amendments of 1988 (CLIA ’88) criteria established for blood lead, i.e., ± 0.19 µmol/L (± 4 µg/dL) or ± 10%, whichever is greater. Performance differences between the strategies were also assessed. We found that 17.7% of all blind PT results were classified as unacceptable compared with only 4.5% of open PT results (P <0.001). In phase 1, 13 of 22 laboratories (60%) exhibited a statistically significant difference (P <0.05) between their blind and open PT performances, although for 6 laboratories the poorer blind performance may not necessarily have led to unsuccessful PT participation under CLIA ’88 criteria. Seven (32%) laboratories had unsuccessful aggregate performance (<80%) under blind testing while maintaining successful performance in open testing. Of these seven, two had gross discrepancies motivating further investigation.
Conclusions: The data suggest that although 
60% of clinical
laboratories make special efforts to improve analytical performance on
open PT samples relative to performance achieved for routine patient
specimens, in most cases the differences are clinically insignificant
and would not likely affect cumulative PT performance. Occasional use
of blind PT may deter the inclination to treat performance samples more
carefully.
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In the US, clinical laboratories are required to participate satisfactorily in a proficiency testing (PT) program for BPb approved under the Clinical Laboratory Improvement Amendments of 1988 (CLIA ’88) (4). The acceptable range for results reported to BPb PT schemes accredited under CLIA ’88 is ± 0.19 µmol/L (± 4 µg/dL) or ± 10% of the established target value (whichever is greater). Before 1992, the acceptable range for BPb was less stringent, ± 0.29 µmol/L (± 6 µg/dL) or ± 15%. This range is still used by the US Department of Labor, Occupational Safety and Health Administration to evaluate the performance of occupational health laboratories providing BPb monitoring services. The more demanding PT performance required under CLIA ’88 is based on a consensus of what modern analytical methods for BPb are capable of producing in terms of accuracy and precision, and are intended to support national and local public health policy toward childhood lead poisoning. Under CLIA ’88, a certified BPb laboratory must report satisfactory results for at least four of five (80%) challenges per test event. Failure to maintain a minimum score of 80% for at least two of three consecutive test events is considered unsuccessful PT performance and can lead to sanctions that may include revocation of the laboratory’s certification to test patient samples. Although under CLIA ’88 the range for acceptable BPb results reported to approved PT schemes is tighter than required previously, it is still more lenient than external quality assessment schemes operated in Canada (Le Centre de Toxicologie du Québec, Programme de Comparisons Interlaboratoires, Laboratorie de Toxicologie-CHUL, 2705 Boul. Laurier, Sainte-Foy, Quebec, Canada G1V-4G2) and in the United Kingdom [Trace Elements External Quality Assessment Scheme (TEQAS), Centre for Clinical Science, School of Biological Sciences, University of Surrey, Guildford, Surrey, United Kingdom GU2 5XH], where good laboratory performance at a BPb concentration of 0.48 µmol/L (10 µg/dL) is expected to be ± 0.05 µmol/L (± 1 µg/dL), and acceptable performance is considered ± 0.15 µmol/L (± 3 µg/dL).
BPb laboratories have improved analytical performance over the last two decades by upgrading their analytical instrumentation and adopting more rigorous internal quality-control practices (5). The role of external PT within the laboratory’s overall quality assurance strategy is generally regarded as useful, but it can be a cause for concern because of the regulatory consequences of unsuccessful performance (6). A valid criticism of current PT program arrangements is that performance samples are not truly blinded; they are clearly identified as PT samples, and shipment schedules are provided to participating laboratories well in advance. Thus, laboratory technical staff are fully cognizant of, and are prepared to analyze, PT performance samples. There may be a tendency to treat PT samples with more care or to analyze them using more experienced personnel, more repeated testing, or with different analytical protocols than routine patient specimens receive.
Under CLIA ’88, the clinical laboratory director and the individuals performing the test are required to sign an attestation statement on the PT program report form that PT samples are tested in the same manner as patient samples (4). Moreover, it is the policy of some state PT programs that repeated testing or analysis of PT samples is not permitted unless the laboratory performs the same repetitive testing or analysis on similar non-PT samples. There has been criticism of some PT programs for providing performance samples/challenges that do not truly resemble patient specimens, for example, when a reconstitution step is required (7). Among the criticisms aimed at current BPb PT program designs is that they test only the analytical component of the process, leaving preanalytical variables untested (8).
In this study, we explore some of these criticisms by examining the effectiveness of the current open PT strategy. We describe a double-blinded assessment of clinical laboratory performance for BPb in which open PT samples, deliberately disguised as routine patient samples, were submitted to up to 42 clinical laboratories certified for this analysis. Thus, the testing laboratory was blinded not only to the BPb concentration, but also to the performance nature of the sample. We report the results of this large-scale assessment and its implications for the traditional open approach to external PT schemes.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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NYSDH BPb PT program.
The NYSDH BPb PT program was established
in the mid-1970s and is currently administered by the NYSDH’s
Wadsworth Center (5). In 1993, the NYSDH program obtained
CLIA ’88-approved PT program provider status. Under federal criteria
for approved status, the program must provide five challenges (samples)
three times per year. PT sample materials are based on whole blood
derived from lead-dosed goats so that lead is "physiologically
bound" within the red blood cells. Sample reconstitution is not
required because whole blood is taken from the animal’s jugular vein,
transferred into blood collection bags, and immediately preserved in
dipotassium EDTA. All blood collection procedures are covered
under an active protocol approved by the appropriate Institutional
Animal Care and Use Committee. Approximately 400–500 mL of blood per
animal are collected, and 3-mL aliquots are dispensed into
additive-free evacuated glass tubes. PT materials are shipped in
styrofoam boxes to participating laboratories as liquid whole-blood
samples without refrigeration. This practice ensures comparability with
shipment of routine patient specimens and avoids any potential matrix
effects that can occur when inorganic lead is added into a blood
matrix. However, the PT samples are clearly identified as such and are
easily recognizable in the laboratory because the additive-free tubes
have red stoppers.
WSLH BPb PT program.
The WSLH BPb PT program was established
with federal grant funding in 1988 after cessation of the CDC PT
program in 1987 (9). The program was modified in 1995 to
provide for three special events in which the number of challenges was
increased from three to five to satisfy federal requirements for
approved status under CLIA ’88. In the WSLH program, lead-dosed cows
are used as the source of PT samples. Routine practice at the WSLH is
to withdraw an 800-mL blood pool from each animal on each of 4
subsequent days following a lead dose. Blood is collected into a blood
bag containing disodium EDTA preservative, and 3-mL samples are
transferred into additive-free (red-stoppered) tubes. These tubes are
stored frozen for subsequent shipment to participating laboratories.
Freezing causes the red blood cells to lyse, and when thawed, the WSLH
PT samples appear very dark rather than bright red in color. In this
form, WSLH PT samples are difficult to disguise as patient samples and
would likely be rejected as a specimen for analysis, or may be
identifiable as a PT sample regardless of the tube identification. To
avoid this problem and to maintain blindness for the WSLH samples,
aliquots of material were transferred into prewashed,
lavender-stoppered tubes for distribution as blind samples. Providing
WSLH PT materials prepared as intact red blood cells, rather than as
lysed material, would not be expected to affect analytical methods
designed to measure lead in whole blood.
The issue of a "species effect", i.e., animal vs human blood, is occasionally raised to explain cases of PT failure. However, there is no direct evidence that modern BPb methods suffer from a species effect. A major advantage in using animal blood is that it avoids potential health hazards associated with distributing human blood of unknown etiology. Animal blood is the most common sample matrix for BPb performance assessment among PT schemes in the US and overseas (10).
Taken together, the NYSDH and WSLH PT programs provide for as many as 57 BPb challenges per year. Target values for BPb in both programs are calculated only after all participant results have been reported and are based on a 90% (or better) consensus of at least 10 referee laboratories using well-established techniques. Both PT programs are also used by the Occupational Safety and Health Administration for accrediting occupational BPb testing laboratories.
study design
This 3-year observational study consisted of two phases. To
achieve sufficient statistical power from a large number of PT
challenges over the study period, phase 1 included 22 certified
clinical laboratories participating in both the NYSDH and WSLH PT
programs for BPb. The second phase was limited to 24 certified clinical
laboratories participating in a single event from the NYSDH PT program.
In phase 1, clinical laboratories were selected for a blinded assessment of laboratory performance in the following manner: A list of laboratories that participate in the NYSDH BPb PT program was matched with one from the WSLH BPb PT program. A total of 41 laboratories were identified as participants in both PT programs. Each laboratory was contacted by one of us (D. J., L. W., H. M., or T. M.) and asked if they would accept specimens for BPb analysis submitted by a physician. Nineteen laboratories (46%) indicated that they were unable to provide laboratory services to individual physicians. The remaining 22 laboratories formed the basis of the phase 1 study population. They included 14 independent, 6 hospital-affiliated, and 2 public health laboratories.
Blinding was accomplished by preparing duplicate sets of PT samples in standard lavender-stoppered (EDTA) blood collection tubes labeled with fictitious patient names. Phase 1 blind PT samples were prepared and distributed as follows: Aliquots (2 mL) of the same PT whole blood were dispensed into lavender-stoppered tubes to ensure that they closely resembled routine venous specimens from patients. The lavender-stoppered tubes were prewashed with doubly deionized water to remove endogenous disodium EDTA anticoagulant because the PT material was already preserved with either dipotassium EDTA (NY) or disodium EDTA (WI) during the collection process. Collection tubes were air-dried in a dust-free cabinet set aside to dry acid-washed laboratory ware that is also used for trace element analyses. Without a prewash step, the endogenous EDTA in the tubes would have doubled the concentration recommended for anodic stripping voltammetry (ASV) techniques, and this would have adversely affected ASV method performance. One laboratory requested that blood specimens be submitted as dried blood spots using their special lead-free filter paper collection kit. To accommodate these specimen requirements, its PT samples were transferred to filter paper sheets, carefully dried in a dust-free cabinet, and submitted for BPb analysis.
Double-blinded PT samples were submitted with laboratory-specific requisition slips by one of four physicians to each respective participant laboratory. Local and state childhood lead-poisoning program personnel were notified of the study in advance and were provided with the fictitious patient names and addresses to prevent any unnecessary public-health concerns or follow-up actions, including the incorporation of fictitious data into public-health database registries. Fictitious patient test results were reported by each testing laboratory to the requesting physician, who forwarded the BPb test results to the study coordinators for comparison with (a) the matching open test results reported directly to the PT program provider by the certified laboratory and (b) the established PT target value for that sample.
In phase 2, the selection criteria were modified to target a different
clinical laboratory population. We limited the selection to
laboratories that analyze capillary whole-blood (micro) specimens for
BPb, and because we confined phase 2 to a single test event in the
NYSDH PT program, we concentrated on identifying laboratories that
participate in the NYSDH PT program but not in the WSLH program. Of
113 potential selectees, we contacted 40 phase 2 laboratories and
asked if they would accept capillary blood specimens for BPb analysis
from a physician’s office. A total of 24 laboratories (60%) responded
positively. These included 1 public health laboratory, 14 independent,
and 9 hospital-affiliated laboratories. PT samples from a single NYSDH
test event were transferred into prewashed, lavender-stoppered, plastic
Microtainer tubes (Becton Dickinson) and, disguised as routine
screening specimens, were submitted to the participant laboratories. As
above, the tubes were prewashed to remove disodium EDTA anticoagulant.
statistical analysis
We examined the discrepancies between blind and open PT results in
two ways. A discrete assessment was made by grading each PT challenge
as either acceptable or unacceptable based on the fixed criteria
allowed under CLIA ’88, i.e., ± 0.19 µmol/L (± 4 µg/dL) or
± 10% around the established target value (whichever was greater). We
then compared the frequency of acceptable results reported for the
blind and open protocols. In this study, it was not possible to follow
each laboratory’s consecutive performance under CLIA ’88 criteria
because the number of test events provided by the WSLH program are more
frequent than required under the CLIA ’88 model. Additionally, most of
the challenges provided by the WSLH program are not graded for CLIA
’88 purposes. Pooling the data from two different programs also
presented logistical difficulties with grading for certification
purposes. Therefore, to provide an indication of cumulative performance
comparable to that required under CLIA ’88 for successful performance,
we used an aggregate score of 80% acceptable responses to define
successful performance for blind and open PT.
A continuous assessment of the data is also desirable because
the use of fixed acceptability criteria leads to a loss of information
when laboratory performance is simply dichotomized as either acceptable
or unacceptable. For the continuous assessment, we chose to compare
root mean squared errors (RMSE), defined as the square roots of the
average differences between reported BPb values and the program target
values, [log (reported result) - log (target
value)]2. A comparison is then possible in terms
of an open PT reliability index, which is defined as a ratio of RMSE
(blind) to RMSE (open), so that values greater than 1 indicate worse
performance when analyzing blind PT samples (Table 1
). Unlike the discrete assessment, which is based on fixed PT
criteria, the continuous assessment may be influenced by outliers
because they inflate the RMSE value.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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When individual test results were graded against CLIA ’88 criteria, we
found that 17.7% of all blind PT results were classified as
unacceptable compared with 4.6% of the open PT results. This
difference is statistically significant (P <0.001,
2 test for symmetry on 1 degree of freedom).
Laboratory-specific performances are shown in Fig. 1
, where blind PT performance shown in the top panel can be
directly compared with open PT performance shown in the bottom panel.
Each column shows the percentage of BPb test results that were graded
as satisfactory. An 80% aggregate performance threshold is shown as a
solid line in both panels. This approximates the minimum cumulative
performance allowed under CLIA ’88 for certified clinical laboratories
performing the analysis for BPb. Each gray column denotes a
laboratory-specific data set for phase 1 with each laboratory
identified by an arbitrary number (1–22). Not all laboratories
participated in the same number of PT events; therefore, the number of
individual challenges or samples tested varied from 5 to 67 during
phase 1, as shown in the top panel of Fig. 1
. Because of the matched
blind-open design of the study, the number of open samples was often
the same (9 laboratories) or within ± 5 (12 laboratories) of the
number of blind samples. However, laboratory 7 had 9 fewer and
laboratory 10 had 32 fewer open samples. The reduced number of open
samples for laboratory 10 accounts for its lack of significance.
Additionally, the performances of within-blind and -open categories
were tested for equality. No significant differences were detected
among the open performances. However, when blinded, laboratories 17,
16, and 22 (as indicated by asterisks in Fig. 1
) had significantly more
unsatisfactory results, and laboratory 19 (as indicated with the
pentagon in Fig. 1
) had significantly more satisfactory results than
the combined results of the other phase 1 laboratories. The pooled data
from all 22 laboratories in phase 1 are shown as a hatched column
(column P1 in Fig. 1
).
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During the course of phase 1, two laboratories ceased to accept patient
specimens because of unsuccessful PT performance, which necessitated
their removal from the blind testing PT study. In phase 2, we elected
to pool all data from 23 laboratories that returned blind results
because the number of challenges in phase 2 was only five for each
laboratory. The phase 2 data were pooled into two groups (shown in Fig. 1
as two hatched columns): one group (n = 20) that did not
participate in phase 1 (column P2) and another group (n = 3) that
participated in both phase 1 and phase 2 (column P1+P2).
Individual differences between blind and open PT performance are shown
in the middle panel of Fig. 1
. Participating laboratories and pooled
data sets are categorically sorted to show increasing discrepancy from
left to right. The statistical significance (P) of the
difference between blind and open PT performance is given in the middle
panel with increasingly poorer performance in blind PT testing compared
with open PT shown from left to right. The middle panel also shows
that, collectively, both phase 1 (column P1) and phase 2 (column P2)
laboratories scored significantly worse on blind challenges than on
open PT challenges. However, they differed significantly from each
other only on their open performances (P = 0.0047).
Thirteen of the 22 phase 1 laboratories (60%) exhibited a
statistically significant difference between their blind and open PT
performance (P <0.05). Seven (32%) failed to obtain an
aggregate performance score of 80% on blind PT samples but maintained
a cumulative performance 
80% for open PT samples. Within this latter
group, two laboratories with the poorest reliability indices (Table 1
)
exhibited a large negative bias under blind testing; therefore, further
investigations were warranted.
In Fig. 2
, difference plots for blind and open PT results are shown for
the best 3 (top panel) and worst 3 (bottom panel) laboratories selected
from the 22 phase 1 participants. The worst three (identified with an
asterisk in Fig. 1
) had significantly more unsatisfactory results than
the combined results of all other phase 1 laboratories. Each plot shows
the deviation between a reported result and the program target value as
a function of increasing BPb target value concentrations in µg/dL.
Matching blind and open result pairs are shown with a line joining the
two data points. To allow judgment of the clinical acceptability of
individual deviations from the established target values, the
acceptable range for PT samples under CLIA ’88 is shown as solid
lines.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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80%) for this test event based on their
blind PT sample scores. The possibility exists that some laboratories may have recognized the blind samples as PT materials. Most PT program designs challenge laboratories across the range of BPb concentrations expected in clinical practice, but increased BPb concentrations are much less prevalent today than they were two decades ago. Therefore, some clinical laboratories may encounter increased BPb concentrations only occasionally, but PT challenges almost always contain such samples. Although the five blinded PT challenges from the NYSDH PT events were separated into groups of two and three and distributed over a 2-week time frame, one phase 1 participant and one phase 2 participant suspected that PT samples were being referred for BPb analysis by another testing laboratory, a practice that is not permitted under the NYSDH program regulations. The phase 1 laboratory rejected the blind samples for analysis and informed the study physician who had submitted them. The phase 2 laboratory reported its suspicions directly to the PT program provider, who then revealed the blind nature of the study. Disclosure of the blind nature of the study to these two laboratories necessitated their elimination from further blind shipments. Thus, a valid criticism of the current study is that we cannot be sure that these two laboratories did not disclose their suspicions to other laboratories, nor can we be absolutely certain that other laboratories did not recognize the blind specimens as PT samples. Such recognition would be expected to diminish the differences between blind and open PT. These difficulties illustrate some of the reasons that blind PT schemes are not conducted routinely, and if implemented in future, such schemes will need to be carefully designed.
In one case, a laboratory (laboratory 21) reported blind PT results with a large negative bias compared with 100% acceptable open PT results. Because of the potential public-health implications, the laboratory was contacted, and a practice of treating PT samples differently was identified. In a second case, another laboratory (laboratory 22) also reported blind PT results with a large negative bias compared with 100% acceptable open PT results in both the NYSDH and WSLH programs. The laboratory was contacted, and again a practice of treating PT samples differently was identified. Both laboratories were dropped from the study because the blinded nature had been disclosed.
Two previous studies examined BPb laboratory performance under conditions where the nature of the performance samples was blinded to the testing facility (12)(13). Sargent et al. (12) submitted nine well-characterized BPb samples disguised as clinical specimens to 14 private and 4 state laboratories. BPb concentrations ranged from 0.43 to 1.59 µmol/L (9–32.9 µg/dL) with six samples below 0.43 µmol/L. These authors found a wide variation in the validity of BPb measurements reported among 18 CLIA-certified laboratories. Although most laboratories performed within the CLIA criteria used by PT programs in the US, clinically important disparities were evident, with one laboratory repeatedly making false-negative misclassifications. They also reported that up to 42% of test results for one sample with a BPb concentration of 0.43 µmol/L (9.0 µg/dL) were misclassified as false positive. Sargent et al. (12) warn clinicians to be wary of laboratory reports that do not bear out their clinical suspicions, and they advise caution in interpreting BPb results that are within ± 0.14 or 0.19 µmol/L (± 3 or 4 µg/dL) of a threshold value. On the basis of the results from our study, we would certainly concur with their advice.
In another study, Jobanputra et al. (13) examined the performance of eight BPb laboratories by submitting specimens from seven human subjects and three bovine reference materials to eight laboratories. The blood specimens were not identified as performance samples; thus, laboratories were blinded to the true objective for analysis. True BPb concentrations in test specimens were established using inductively coupled plasma mass spectrometry with isotope dilution. The BPb concentrations in all of human specimens were <0.48 µmol/L (10 µg/dL). The BPb concentrations in the three bovine samples ranged from 0.26 to 0.79 µmol/L (5.4–16.4 µg/dL). All eight laboratories reported results within 0.14 µmol/L (3 µg/dL), and there was strong reproducibility within and among laboratories, suggesting adequate accuracy and precision in this group. Only one laboratory exhibited unsatisfactory performance. The limitations of the study include its small sample size and the clustering of BPb concentrations around the 0.48 µmol/L (10 µg/dL) CDC threshold. Because clinical decisions and public-health follow-up actions are based on BPb concentration thresholds of 0.97, 2.12, and 3.38 µmol/L (20, 44, and 70 µg/dL), it is important that BPb laboratory performance also be assessed at these concentrations.
Assessment of PT programs in other areas of clinical laboratory medicine has been reported, but few studies have used blinded strategies. Richardson et al. (14) examined improvements in the quality of diagnostic microbiology over a 20-year period in Ontario through a peer-group proficiency assessment program. They compared results from an open PT program with a retrospective examination of routine work on patient specimens. They found that poorly performing laboratories lacked effective quality control, used nonstandard methods, and failed to follow in-house standard operating procedure manuals. Black and co-workers (15)(16) used a blind split-sample study design to examine the performance of bacteriology laboratories. They reported 94% correct identifications of mucoid Escherichia coli under an open PT design but only 47% under blinded conditions. Significant variations were also found for other microorganisms, suggesting that performance in open PT schemes has little to do with performance on real patient specimens. In a recent retrospective blind study design, Reilly et al. (17) compared laboratory performance in an open PT program for mycology with blind performance assessed against previously tested and reported patient specimens. They reported differential misclassification rates between open and blind PT performances for several organisms.
The design of current open PT schemes is largely driven by logistics and cost and, although not a perfect predictor of true laboratory performance, they are useful educational tools. To our knowledge, this is the first account in which PT samples have been distributed in parallel under both blind and open strategies to clinical laboratories certified under CLIA ’88. The study provides convincing evidence that, at least for a small, nonrandom selection of clinical laboratories that provide BPb testing, some effort is made to improve analytical performance in a manner that is absent for routine patient specimens. This may be possible because PT sample shipment dates are known well in advance, enabling laboratories to schedule instrument calibration and maintenance activities to coincide with arrival of PT samples. In addition, the amount of PT sample provided usually is sufficient to permit multiple testing on different instruments to provide a more robust estimate for reporting.
In conclusion, in our study of 22 laboratories in phase 1,
13 (60%) exhibited a statistically significant difference
(P <0.05) between their blind and open PT performances,
although for 6 laboratories their poorer blind performance may not
necessarily have led to unsuccessful PT participation under CLIA ’88
criteria. Seven (32%) exhibited a highly significant improvement
(P 
0.001) in aggregate performance under open testing
that could be considered equivalent to successful (80%) PT
performance under CLIA ’88 but which could not be confirmed under
blind testing. Of these seven, two had gross discrepancies, motivating
further investigation. The data suggest that although 60% of
clinical laboratories make special efforts to improve analytical
performance on open PT samples relative to performance achieved for
routine patient specimens, in most cases the differences are clinically
insignificant and would not likely change PT accreditation. Occasional
use of blind PT may deter the inclination to treat performance samples
more carefully.
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Acknowledgments |
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Footnotes |
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1 Current address: Division of General Pediatrics, Schneider
Children’s Hospital, Suite 108, New Hyde Park, NY 11042. 
2 Current address: New York Academy of Medicine, Center for Urban
Epidemiologic Studies, 1216 Fifth Ave., New York, NY 10029-5293.
3 Nonstandard abbreviations: BPb, blood lead; PT, proficiency testing; CLIA ’88, Clinical Laboratory Improvement Amendments of 1988; NYSDH, New York State Department of Health; WSLH, Wisconsin State Laboratory of Hygiene; ASV, anodic stripping voltammetry; RMSE, root mean squared error; and ETAAS, electrothermal atomization atomic absorption spectrometry.
4 In the US, BPb concentrations are widely reported as µg/dL. To convert µg/dL into SI units (µmol/L), multiply by 0.04826. To convert µg/L into µmol/L, multiply by 0.004862.
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References |
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)
and blind (•) PT results for 6 of the 44 participant laboratories.