The use of a single non-timed urine collection in the calculation of fractional lithium clearance.

The fractional clearance of lithium (CLLi/CLCR) calculated in 15 normal healthy adults from a single morning urine aliquot collection, together with a single venous blood sample (FQ) was compared with the average CLLi/CLCR obtained from three timed consecutive urine collections with mid-point blood sampling (FABC). Lithium had been ingested 15-18 h prior to the collection of these samples. Mean CLLi/CLCR was similar (FQ, 0.186, FABC, 0.177), with a highly significant correlation in each individual of CLLi/CLCR measured by either method (r = 0.97, P less than 0.0001). Proximal tubular reabsorption of sodium using lithium clearance may be calculated from a single urine and blood sample.