Evaluation of an inductively coupled plasma mass spectrometry method for the analysis of sweat chloride and sodium for use in the diagnosis of cystic fibrosis.

BACKGROUND: Cystic fibrosis (CF) is an autosomal recessive condition that has an incidence of 1:2500 live births in Northern Europe. Due to the large number of mutations that can result in classical or atypical CF phenotype, the sweat test, which quantifies the amount of chloride and sodium in sweat, is vital in supporting the diagnosis of CF. Patients with CF have raised concentrations of chloride and sodium in their sweat; however, it is the concentration of chloride in sweat which provides the greatest diagnostic sensitivity for CF. METHOD: An inductively coupled plasma mass spectrometry (ICP-MS) method for the analysis of sweat chloride and sodium was evaluated for the routine measurement of sweat collected using the Wescor Macroduct(®) Sweat Collection System. The precision, linearity and agreement with the all laboratories trimmed means (ALTMs) and 'weighed-in' concentrations of sodium and chloride in samples supplied by the UK NEQAS external quality assessment (EQA) Sweat Testing Scheme were assessed. RESULTS: This ICP-MS method for the quantification of chloride and sodium in sweat samples was shown to be accurate, precise and suitable for the routine analysis of sweat chloride and sodium. CONCLUSION: The method performs well and is now used in the routine analysis of sweat in this laboratory.

Development of an automatic laboratory computer flagging system to identify urine albumin samples potentially affected by antigen excess ('hooking').

BACKGROUND: Immunoassays for urinary albumin are often subject to the problem of antigen excess (the 'hook' effect) at high albumin concentrations. We developed an automated protocol to identify such samples based on urinary albumin to creatinine ratio (uACR) and urinary total protein (uTP) results. METHODS: An automated flagging system was designed and written into the laboratory computer system to alert technical staff to samples potentially affected by the 'hook effect'. This flag was activated when there was a combination of an uTP of ≥2400 mg/L and an uACR of <30 mg/mmol. RESULTS: The potential rate of false-negative uACR results was approximately 0.17% in samples from primary care and diabetic clinic sources. CONCLUSIONS: Samples with falsely low uACR results were identified, allowing the vast majority of results to be authorized without intervention. The protocol prevented the reporting of false-negative uACR results which might impact on the management of patients.