EXTENDED MEASUREMENT OF GLOMERULAR-FILTRATION RATE AND EFFECTIVE RENAL PLASMA-FLOW IN AMBULATORY PATIENTS

We describe a standardized clearance method over 5 h (one hour equilib ration followed by eight consecutive 30 min clearance periods [period 2-9]) for the estimation of GFR (iothalamate I-125) and ERPF (hippuran I-131) during water diuresis in ambulatory and exercising patients. F our groups were examined. In group I (normal controls, n = 15) there w ere no significant changes in GFR, ERPF and FF (P > 0.10) during repea ted clearance periods (mean of period 2-5 versus period 6-9). The repr oducibility of the method was studied at a mean interval of 3.7 weeks in a group of patients with stable reduction of GFR (group Il, n = 7). The values for GFR, ERPF, FF and RVR did not change significantly in this group, and correlated significantly between repeated studies (r = 0.81 to r = 0.99). In group III (untreated hypertensive patients with reduced GFR, n = 13) there was a time dependent 7.2% decrease in GFR (P < 0.05), significantly different from group I (P < 0.02), a 10.0% d ecrease in ERPF (P < 0.01) and no significant change in FF (P = 0.08) when the mean of period 2-5 was compared with the mean of period 6-9. In healthy controls (group IV, n = 8) light sustained bicycle exercise (25 W) induced a 7.1% decline in GFR (P < 0.01), 17.4% decline in ERP F (P < 0.001) and a 13.6% increase in FF (P < 0.001). We conclude that ambulatory measurements of GFR and RPF can be carried out over a peri od of 5 h with satisfactory precision and repeatability. Ambulatory hy pertensive patients with moderately reduced GFR showed the same degree of time dependent downward drift of GFR and ERPF without exercise as was seen in healthy individuals during light exercise. Accordingly, in these groups single clearance periods imply a risk for under or overe stimation of renal function, and time controls are necessary during cl earance studies.