Evaluation of the Performance of a Micromethod for Measuring Urinary Iodine by Using Six Sigma Quality Metrics

BACKGROUND The urinary iodine micromethod (UIMM) is a modification of the conventional method and its performance needs evaluation. METHODS UIMM performance was evaluated using the method validation and 2008 Iodine Deficiency Disorders survey data obtained from four urinary iodine (UI) laboratories. Method acceptability tests and Sigma quality metrics were determined using total allowable errors (TEas) set by two external quality assurance (EQA) providers. RESULTS UIMM obeyed various method acceptability test criteria with some discrepancies at low concentrations. Method validation data calculated against the UI Quality Program (TUIQP) TEas showed that the Sigma metrics were at 2.75, 1.80, and 3.80 for 51±15.50 µg/L, 108±32.40 µg/L, and 149±38.60 µg/L UI, respectively. External quality control (EQC) data showed that the performance of the laboratories was within Sigma metrics of 0.85-1.12, 1.57-4.36, and 1.46-4.98 at 46.91±7.05 µg/L, 135.14±13.53 µg/L, and 238.58±17.90 µg/L, respectively. No laboratory showed a calculated total error (TEcalc)<total allowable error (TEa) for the low concentration level; all laboratories showed an acceptable performance for the medium-high level, and two laboratories showed an acceptable performance for the high level. When calculated against the Ensuring the Quality of UI Procedures (EQUIP) TEas, the performance of all laboratories was≤2.49 Sigma metrics at all concentrations. Only one laboratory had TEcalc<TEa for the medium-high and high concentrations. CONCLUSIONS UIMM showed unacceptable performance for the iodine deficiency levels and variable performance at other concentrations according to different TEas.