Usefulness of iodine/creatinine ratio from spot-urine samples to evaluate the effectiveness of low-iodine diet preparation for radioiodine therapy

Objective The success of a low-iodine diet (LID) is best determined by measurement of 24-h urine iodine (U-I) excretion. The aim of this study was to determine reliable estimates for 24-h U-I based on spot-urine samples and to provide cut-offs to determine the effectiveness of LID preparation. Design We prospectively measured iodine levels in 193 patients based on 24-h- and spot-urine samples before radioactive iodine therapy. The iodine was expressed as the 24-h U-I excretion (μg/day) and as two different indices from spot urine, simple iodine concentration (simple I) and the iodine/creatinine (I/Cr) ratio. Poor LID preparation was defined as I excretion of >150 μg/day according to the 24-h U-I measurement. Results The measured 24-h U-I was significantly higher than the two indices from spot urine (P < 0·001). However, there were statistically significant correlations between the 24-h U-I values and the two spot-urine-based indices; the correlation coefficient was 0·539 for simple I and 0·773 for I/Cr ratio (P < 0·001). The cut-off of I/Cr ratio for poor LID preparation was >66·2 μg/g Cr (sensitivity 96·4%, specificity 83·6%, positive predictive value 50·0% and negative predictive value 99·3%). Conclusions We demonstrated that the I/Cr ratio from spot urine could serve as a useful and reliable alternative to 24-h urine collection as it has acceptable diagnostic values for detecting poor LID preparation.