A general equation for estimating glomerular filtration rate from a single plasma sample

OBJECTIVES: Glomerular filtration rate (GFR) may be estimated from a single plasma sample measurement using empirical equations. This method forms the basis of international guidelines on GFR measurement. New guidelines have recently been recommended by the British Nuclear Medicine Society (BNMS). These use the slope-intercept technique in which several samples are obtained. Quality control of measurement may be achieved by comparison with the individual single-sample values. This paper compares international guideline single-sample estimates of GFR with the new BNMS method and derives an improved general single-sample equation. METHODS: GFR measurements using Tc-DTPA were performed on 180 adults and 100 children using samples at approximately 2 h and 3 h and a further 45 adults with samples at 2, 3 and 4 h. The two-sample values obtained using the BNMS guideline method were compared to one-sample estimates obtained using (1) international guidelines and (2) a new equation derived from the data. The new equation was evaluated in a further 145 subjects. RESULTS: The international guidelines technique had systematic differences between the one-sample and two-sample estimates of GFR. The new equation had minimal systematic error and reduced random error (standard error of the estimate 4.5 ml . min per 1.73 m). CONCLUSIONS: There were significant differences between GFR values obtained using the international guidelines single-sample method and the slope-intercept method described in the BNMS guidelines. The new equation described in this paper gave considerably improved agreement and is recommended if single-sample estimates are to be used as quality control for BNMS guideline measurements.