Urine test strips and iodine contamination: a tricky trick in iodine nutrition surveys

Objective A recent report from the Faroe Islands suggested mild iodine deficiency among women aged 40 years and older. New preliminary results showed an average urinary iodine concentration of 457 mu g/L. This spurious finding encouraged the present report. Methods A subset of 17 consecutive pregnant women from an ongoing study provided an additional morning spot urine sample; one part was tested for a few seconds with a test strip for glucose, protein, and erythrocytes, and the other was left untouched. The women recorded the previous week's intake of iodine-rich foods. Additionally, 12 tap water samples were collected such that three samples were sealed immediately, the remaining were strip tested for 10, 30 and 60 s, respectively. Urine and water samples were analysed using the ceri/arsen method after alkaline ashing. Results Median urinary iodine concentration (UIC) in pristine urine samples was 116 mu g/L (IQR 79-204 mu g/L; range 26-475 mu g/L), and positively associated with iodine-rich food intake. UIC increase with test-strip dip varied from 10 to 94 times the non-dip value. In tap water, the pristine samples had an iodine concentration of 2-4 mu g/L, which increased to 10,000 mu g/L after 60 s. Conclusion Urine samples exposed to test strips can be contaminated within a few seconds leading to unreliable iodine results. Therefore, it is crucial that the study protocol clearly states the procedure for urine sample collection and handling, strict adherence to protocol, and that utensils used are dedicated to collecting urine to measure iodine.