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.


METHODS
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 (microg/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 microg/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 microg/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.
Kim
Hee Kyung
HK
Division of Endocrinology and Metabolism, Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, 50 Irwon-dong, Gangnam-gu, Seoul, Korea.
Lee
Soo Youn
SY
Lee
Ji In
JI
Jang
Hye Won
HW
Kim
Soo Kyoung
SK
Chung
Hye Soo
HS
Tan
Alice Hyun Kyung
AH
Hur
Kyu Yeon
KY
Kim
Jae Hyeon
JH
Chung
Jae Hoon
JH
Kim
Sun Wook
SW
D000328:Adult; D003404:Creatinine / Q000652:urine*; D004032:Diet; D005260:Female; D006801:Humans; D007455:Iodine / Q000008:administration & dosage / Q000652:urine*; D007457:Iodine Radioisotopes / Q000627:therapeutic use*; D008297:Male; D008875:Middle Aged; D016482:Urinalysis / Q000379:methods
D007457:Iodine Radioisotopes; D007455:Iodine; D003404:Creatinine