Effect of truncating blood sampling in measuring glomerular filtration rate.

Glomerular filtration rates (GFR's) are used to guide patient management. GFR's are based on radioactivity measurements of blood samples sampled at different times. We compared GFR computations from blood collected at 6 times to those collected at 2 timepoints. Thirty-seven GFR studies were performed on 25 patients. After intravenous administration of I-125 sodium iothalamate, 6 plasma samples were obtained at 5 min, 10 min, 15 min, 3 hr, 3.5 hr and 4 hr after injection, then counted in a well counter. Two different GFR calculation tools were applied to each set of 6 plasma counts (Methods 1 and 2), and a 2-sample algorithm (Method 3) computed GFR using only 3 hr and 4 hr data. Linear correlation between Method 1 and 2 GFR's was stronger than for Method 3 versus Methods 1 and 2 (r = 1.00 versus r = .91, P < .0001). Bland-Altman limits of agreement were larger (P < .0001) for Method 3 versus Methods 1 and 2 (-39.5 to +22.0 ml/min/1.73 m²) than for Method 1 versus 2 (-7.6 to +4.5 ml/min/1.73 m²). Method 3 overestimated lower GFR's and underestimated higher GFR's. Methods 1 and 2 agreed exactly in identifying 3 cases of GFR < 74 ml/min/1.73 m² (κ = 1.00), while Method 3 detected only 1 of the three (κ = .48). To avoid underdiagnosing low GFR's, larger GFR sample sizes are preferable to smaller sample sizes.