Sample Size and Optimal Timing for Urine Collection Based on Diurnal Variability of Urinary Iodine/Creatinine Ratio in Pregnant Women.

Urinary iodine concentration (UIC) is influenced by variable urine volume, undermining its reliability. The urinary iodine/creatinine ratio (UI/Cr) may provide a more stable measure, especially in pregnancy. This study aimed to explore the variation of UI/Cr in assessing iodine status in pregnant women, and determine the sample size and optimal sampling time for accurate assessment. Thirteen pregnant women participated in a seven-day iodine metabolism study. All urine samples from each participant were collected separately over the 7-day period, and urinary iodine and urinary creatinine were tested. Spot UIC was positively correlated with time interval (β = 12.09, P < 0.001). UI/Cr, an indicator corrected for volume effect, showed no significant effect with time interval (β = -3.00, P = 0.090). The CV of UI/Cr was lower than UIC at both individual (56.16% vs. 73.15%) and population levels (62.41% vs. 84.75%). With a 95% confidence interval and ± 20% precision, an individual needs 33 urine samples, while a population requires 39 samples. The mixed-effects model analysis revealed that spot UI/Cr during the 18:00-23:59 period exhibited the strongest correlation with 24-hour UI/Cr (β = 0.62, P < 0.001). Additionally, spot UI/Cr during the 12:00-17:59 period demonstrated the strongest correlation with 24-hour urinary iodine excretion (β = 0.61, P < 0.001). The UI/Cr provides a more reliable estimate of iodine status than UIC. To obtain a precision of ± 20%, 33 spot-urine samples are required for individual assessment and 39 for population-level evaluation. We recommend urine collection between 12:00 and 18:00 for accurate evaluation.