Analysis of a novel field dilution method for testing samples that exceed the analytic range of point-of-care blood lead analyzers

Figures & data

Figure 1. Field LCII vs. ICP-MS values with ±10% error bounds from ICP-MS results^*. Triangle: Field LCII vs. ICP-MS values. Dashed lines: ±10% of ICP-MS value. Note: ^*One point is excluded from this chart for the sake of clarity. It had an ICP-MS result of 286 µg/dL with a Field LCII result of 262.5 µg/dL. This value was within ±10% of the ICP-MS values.

Figure 2. Difference between Field LCII and ICP-MS value as measured against ICP-MS^*: Triangle: Field LCII-ICP-MS value. Dashed lines: 95% Level of agreement (−14.6, +25.7). Dot-dash line: Average difference Field LCII – ICP-MS (+5.5). Note: ^*One point is excluded from this chart as it appeared to be a reporting error. This point had an ICP-MS result of 694 µg/dL with a bias of −234.1.

Figure 3. CDC LCII vs. ICP-MS values with ±10% error bounds from ICP-MS results. Triangle: CDC LCII vs. ICP-MS values. Dashed lines: ±10% of ICP-MS value.

Figure 4. Difference between CDC LCII and ICP-MS value as measured against ICP-MS. Triangle: Field LCII-ICP-MS value. Dashed lines: 95% Level of agreement (−7.9, +23.5). Dot-dash line: Average difference Field LCII – ICP-MS (+7.8).

Table 1. CCC results comparing each site vs. ICP-MS as well as to each other.

Comparison	n	CCC	95% CI
CDC LCII vs. ICP-MS	211	0.976	(0.970, 0.981)
Field LCII vs. ICP-MS	68	0.910	(0.861, 0.942)
CDC LCII vs. Field LCII	53	0.721	(0.565, 0.827)

Table 2. Number and percent of CDC and Field LCII dilution procedure values within ±10% of the ICP-MS value by ICP-MS quartile.

ICP-MS groups (μg/dL) (n dilution samples)	Average CDC LCII dilution bias (μg/dL) (% difference vs. mean ICP-MS value)	Range of values (μg/dL)	Number of values within ±10% of ICP-MS value (%)
CDC LCII
≤ 66.6 (n = 55)	5.9 (10%)	−5.2–21.4	31 (56%)^a
66.7–77.5 (n = 52)	5.1 (7%)	−8.9–15.7	35 (67%)
77.6–94.7 (n = 51)	8.0 (9%)	−6.6–23.2	25 (49%)
> 94.7 (n = 53)	12.3 (7%)	−12.0–44.0	36 (68%)
All CDC LCII samples (n = 211)	7.8 (8%)	−12.0–44.0	127 (60%)
Field LCII
≤ 66.6 (n = 11)	8.3 (13%)	−7.0–27.7	4 (36%)^a
66.7–77.5 (n = 24)	7.0 (10%)	−8.3–21.3	12 (50%)
77.6–94.7 (n = 26)	4.7 (5%)	−15.5–36.0	15 (58%)
> 94.7 (n = 7)	9.5 (9%)	−23.5–13.1	6 (86%)^b
All Field LCII samples (n = 68)	5.5 (7%)	−23.5–36.0	37 (54%)^b

^a Χ² for equal proportions among categories ( p = 0.9994 for CDC LCII and p = 0.9932 for Field LCII).

^b One Field LCII dilution result was outside of the analytic range of LCII, even when using the dilution method, and was excluded. This field value was 459.9 μg/dL and ICP-MS value was 694.0 μg/dL. If this value is included in the analysis, results for >94.7 (n = 8) are dilution −29.9 (18%), range (−234.1–13.1), 6 within ±10% (75%). For overall results, n = 69, 2.1 bias (2%), range (−234.1–36.0), 37 within ±10% (54%).

Table

Abbreviations
BLL	Blood lead level
CDC	Centers for Disease Control and Prevention
CLIA	Clinical Laboratory Improvement Amendments
ICP-MS	Inductively Coupled Plasma-Mass Spectrometry
LCII	LeadCare II analyzer
MSF	Médecins Sans Frontières
WHO	World Health Organization