Development of an outpatient finger-prick glomerular filtration rate (GFR) procedure suitable for epidemiological studies. In clinical practice, reference GFR procedures are rarely used; in large-scale research studies, a great deal of effort and experience is required to obtain them, which is a considerable disincentive to using GFR as an end point. The major problem for both clinical staff and the subject is the length of time that the procedure takes, requiring continuous attendance in the outpatient clinic or its vicinity. Using iohexol as a marker, we therefore propose an alternative approach, which addresses this fundamental deterrent to a more widespread use of GFR measurement. Eighty-two GFR measurements were performed in a mixture of healthy subjects and patients with differing degrees of renal impairment with a wide range of GFRs. Serum was obtained from blood samples to enable a reference GFR to be calculated. Blood spots were collected on filter paper at the same intervals (120, 180, and 240 min), allowed to dry, and then sent through the post. Serum and blood spots were analyzed simultaneously for each individual by automated reverse-phase high-pressure liquid chromatography. Standard linear regression analyses confirmed a good agreement (r2 = 0.953) between the iohexol serum GFR and iohexol blood spots GFR. Bland-Altman analysis confirmed that there was no concentration bias. Paired comparisons (Wilcoxon's paired signed rank test) showed no significant difference between the two measurements. Capillary sampling is simple, effective, and significantly reduces the time and costs of performing plasma clearance GFR measurements. This approach will make the GFR measurement more accessible for clinical practice and large-scale epidemiological studies may become feasible.