Sara P. Wyness

Performance characteristics of six automated 25-hydroxyvitamin D assays: Mind your 3s and 2s.

BACKGROUND Measurement of 25(OH)D has evolved rapidly, with an increased number of high-throughput automated immunoassays becoming available in recent years. The aim of this study was to fully evaluate six commercially available automated 25(OH)D immunoassays, including 2 newly available (Beckman Coulter) and one recalibrated (Siemens) assay. Comparisons were made to specifically identify the effect of the absence or presence of 25(OH)D2 on these assays. DESIGN AND METHODS Access2 and UniCel DxI 800 (Beckman Coulter), ARCHITECT i2000SR (Abbott Diagnostics), ADVIA Centaur XP (Siemens), Liaison XL (DiaSorin) and MODULAR E170 (Roche Diagnostics) assays were assessed for accuracy, imprecision, interference, limit of blank, and linearity. All were compared to an in-house LC-MS/MS method (traceable to NIST SRM 972) using Passing-Bablok regression and Bland-Altman bias plots. Method comparisons used residual serum samples with both endogenous 25(OH)D2 and 25(OH)D3 (n=50) or 25(OH)D3 only (n=86). Comparisons with all 136 samples were intended to simulate real-world laboratory testing. RESULTS The majority of assays under-recovered 25(OH)D in comparison to LC-MS/MS, with three of six immunoassays affected by the presence of 25(OH)D2. Imprecision was greatest at 25(OH)D concentrations near the decision limits used to assess deficiency. Only two of six immunoassays would meet the recommended bias criteria of <5%. CONCLUSIONS Although standardization efforts continue, these differences in performance remain a concern. Clinicians should be aware when comparing results among assays and using them to determine adequacy of 25(OH)D stores in various patient populations.

Human chorionic gonadotropin discriminatory zone in ectopic pregnancy: does assay harmonization matter?

OBJECTIVE To determine the effect that lack of hCG assay harmonization has on the interpretation of a serum hCG concentration with regards to the hCG discriminatory zone. DESIGN A multisite method comparison study. SETTING Clinical laboratories. PATIENT(S) Eighty serum samples containing various concentrations of hCG. INTERVENTION(S) None. MAIN OUTCOME MEASURE(S) Concentrations of hCG obtained from seven hCG reagent platforms. RESULT(S) The hCG concentrations were significantly different across hCG reagent platforms. Seventy-one percent of assay pairs showed significant differences with samples selected based on hCG concentrations between 1,500 and 3,500 IU/L as determined by a comparative method. Relative to the comparative method, the calculated hCG discriminatory zones for five assays were within 9%, and one assay was within 40% of the target concentrations of 1,500 and 3,500 IU/L. CONCLUSION(S) Despite significant differences in hCG concentrations across hCG immunoassays, an hCG concentration within a discriminatory zone of 1,500-3,500 IU/L can be used for all but one commonly used hCG reagent platform.

Clinical performance evaluation of total protein measurement by digital refractometry and characterization of non-protein solute interferences

Objectives Refractometric methods to measure total protein (TP) in serum and plasma specimens have been replaced by automated biuret methods in virtually all routine clinical testing. A subset of laboratories, however, still report using refractometry to measure TP in conjunction with serum protein electrophoresis. The objective of this study was therefore to conduct a modern performance evaluation of a digital refractometer for TP measurement. Design and methods Performance evaluation of a MISCO Palm Abbe™ digital refractometer was conducted through device familiarization, carryover, precision, accuracy, linearity, analytical sensitivity, analytical specificity, and reference interval verification. Comparison assays included a manual refractometer and an automated biuret assay. Results Carryover risk was eliminated using a demineralized distilled water (ddH2O) wash step. Precision studies demonstrated overall imprecision of 2.2% CV (low TP pool) and 0.5% CV (high TP pool). Accuracy studies demonstrated correlation to both manual refractometry and the biuret method. An overall positive bias (+5.0%) was observed versus the biuret method. On average, outlier specimens had an increased triglyceride concentration. Linearity was verified using mixed dilutions of: a) low and high concentration patient pools, or b) albumin-spiked ddH2O and high concentration patient pool. Decreased recovery was observed using ddH2O dilutions at low TP concentrations. Significant interference was detected at high concentrations of glucose (>267 mg/dL) and triglycerides (>580 mg/dL). Current laboratory reference intervals for TP were verified. Conclusions Performance characteristics of this digital refractometer were validated in a clinical laboratory setting. Biuret method remains the preferred assay for TP measurement in routine clinical analyses.

Multiple macroenzymes in a patient with AIDS: diagnosis using ultrafiltration.

OBJECTIVES Multiple immunoglobulin-bound enzymes (macroenzymes) are reported for the first time in an individual with AIDS. Possible causes and suitable methods of detection are addressed. METHODS An asymptomatic man with a history of AIDS with hypergammaglobulinemia and elevated creatine kinase, amylase, and liver enzyme concentrations was evaluated before enrollment in a clinical trial. Macroenzymes were considered a possible source of these elevated concentrations. RESULTS Polyethylene glycol (PEG) precipitation and ultrafiltration (UF) were used to evaluate the presence of seven macroenzymes. PEG results suggested the presence of six of seven macroenzymes tested, while UF revealed three. UF results supported the clinical presentation. CONCLUSIONS A previous report shows that in cases of excess immunoglobulin, PEG coprecipitates monomeric enzymes along with serum globulins, causing false-positive reporting of macroenzymes. This may explain the discrepancy between PEG and UF results in the presence of hypergammaglobulinemia, making UF a better method of detection in these circumstances.

Evaluation and analytical validation of a handheld digital refractometer for urine specific gravity measurement

Objectives Refractometers are commonly used to determine urine specific gravity (SG) in the assessment of hydration status and urine specimen validity testing. Few comprehensive performance evaluations are available demonstrating refractometer capability from a clinical laboratory perspective. The objective of this study was therefore to conduct an analytical validation of a handheld digital refractometer used for human urine SG testing. Design and methods A MISCO Palm Abbe™ refractometer was used for all experiments, including device familiarization, carryover, precision, accuracy, linearity, analytical sensitivity, evaluation of potential substances which contribute to SG (i.e. “interference”), and reference interval evaluation. A manual refractometer, urine osmometer, and a solute score (sum of urine chloride, creatinine, glucose, potassium, sodium, total protein, and urea nitrogen; all in mg/dL) were used as comparative methods for accuracy assessment. Results Significant carryover was not observed. A wash step was still included as good laboratory practice. Low imprecision (%CV, <0.01) was demonstrated using low and high QC material. Accuracy studies showed strong correlation to manual refractometry. Linear correlation was also demonstrated between SG, osmolality, and solute score. Linearity of Palm Abbe performance was verified with observed error of ≤0.1%. Increases in SG were observed with increasing concentrations of albumin, creatinine, glucose, hemoglobin, sodium chloride, and urea. Transference of a previously published urine SG reference interval of 1.0020–1.0300 was validated. Conclusions The Palm Abbe digital refractometer was a fast, simple, and accurate way to measure urine SG. Analytical validity was confirmed by the present experiments.

Lipemic interference of ceruloplasmin assays – An evaluation of lipid removal methods

BACKGROUND The present studies were conducted to characterize lipemic interference across three FDA-cleared ceruloplasmin (CERU) assays and to evaluate procedures designed to remove lipemic interference. METHODS CERU assays on the Abbott ARCHITECT ci8200, Beckman AU5800, and Roche cobas Integra 400 Plus were evaluated. Precision, linearity with dilution, lipemic interference, and three methods for removing lipemia were assessed on each platform: ultracentrifugation (UC), lipemia-clearing reagent LipoClear (LC), and 1:5 dilution (DIL). Lipemia-index (L-index) thresholds were established using endogenously lipemic specimens and sera spiked with human-derived triglyceride-rich lipoproteins. RESULTS The ci8200 showed greater susceptibility to endogenous lipemic interference than would be expected based on vendor-derived limits established with Intralipid. Endogenous lipemia causes a negative interference on the ci8200 and a positive interference on the Integra. UC was generally the most reliable method of removing lipemic interference without impacting baseline CERU results. CONCLUSIONS CERU assays on different platforms have varying susceptibility to lipemic interference. L-index thresholds derived using Intralipid may not accurately represent interference caused by endogenous lipemia.