Creatinine as a normalization factor to estimate the representativeness of urine sample - Intra-subject and inter-subject variability studies

In-vitro bioassay monitoring generally involves analysis of overnight urine samples (~12 h) collected from radiation workers to estimate the excretion rate of radionuclides from the body. The unknown duration of sample collection (10-16 h) adds to the overall uncertainty in computation of internal dose. In order to minimize this, IAEA recommends measurement of specific gravity or creatinine excretion rate in urine. Creatinine is excreted at a steady rate with normally functioning kidneys therefore, can be used as a normalization factor to infer the duration of collection and/or dilution of the sample, if any. The present study reports the chemical procedure standardized and its application for the estimation of creatinine as well as creatinine co-efficient in normal healthy individuals. Observations indicate higher inter-subject variability and lower constancy in daily excretion of creatinine for the same subject. Thus creatinine excretion rate may not be a useful indicator for extrapolating to 24 h sample collection.