Creatinine adjustment of spot urine samples and 24 h excretion of iodine, selenium, perchlorate, and thiocyanate.

Creatinine (CR) adjustment is widely used for the estimation of urinary 24 h excretion from spot urine samples. We have compared CR-adjusted values for urinary iodine, selenium, perchlorate, and thiocyanate to measured 24 h excretion. The urine samples were collected from a cohort of 14 breastfeeding mothers with both spot samples and 24 h collection, 52 24 h and spot sample pairs where the 24 h CR value fell within the "normal" adult female CR excretion range of 0.6-1.6 g/day were considered for this analysis. In addition, a nonlactating female and a male subject provided all micturitions for 1 and 5 days, respectively. Creatinine was analyzed with a Jáffe reaction-based automated analyzer. Iodine and selenium were determined with induction coupled plasma-mass spectrometry (ICP-MS). Perchlorate and thiocyanate were measured with ion chromatography (IC)-isotope dilution tandem mass spectrometry (MS/MS). Creatinine-adjusted values were poor substitutes of the actual 24 h excretion values (average deviation +/-69, 78, 105, and 104% for iodine, selenium, perchlorate, and thiocyanate, respectively.). Over a 5 day period, the 24 h iodine excretion predicted based on creatinine adjustment of spot samples for the same individual deviated between -83.5 to +101% from the actual measured value, the minimum absolute error being 2.5%. Creatinine adjustment for estimation of 24 h excretion from spot samples was not effective for iodine, selenium, perchlorate, or thiocyanate.