Renal function in the elderly (>70 years old) measured by means of iohexol clearance, serum creatinine, serum urea and estimated clearance

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Abstract
Objective: A survey revealed that kidney donors live longer than non‐donors. When measured using an injection technique in elderly former kidney donors, the glomerular filtration rate (GFR) was found to be markedly decreased. The aim of this study was to determine the relationship between GFR and age in a cohort of elderly controls with two kidneys. We were also interested to find out whether GFR estimated using different equations correlated with the exact measurements. Material and Methods: Renal function was determined using the iohexol technique in 52 elderly “healthy” persons aged 70–110 years. Blood tests were done at the same time. Estimated clearance was determined using the equations of Cockroft–Gault, Walser and Levey. Results: GFR showed a strong correlation with age (p = 0.0002), with an annual decline of 1.05 ml/min. Using formulas for estimation of clearance, the best correlation was found with that of Levey. Probably the most widely‐used formula, that of Cockroft–Gault, underestimated the clearance, the mean value being 46.2 ± 11.3 ml/min/1.73 m 2 , compared to the measured mean value of 67.7 ± 10.8 ml/min/1.73 m 2 . Serum (s)‐creatinine did not correlate with age (p = 0.3997). However, s‐urea increased with age (p = 0.0019), while s‐albumin (p = 0.0018), blood haemoglobin (p = 0.0060) and s‐ferritin (p = 0.0243) decreased with age. A total of 21/52 (30%) of the elderly subjects had a GFR >70 ml/min/1.73 m 2 , but none of these persons was aged >90 years. Of the 21 subjects with good function, eight seemed to be quite healthy, with normal test values. Conclusion: GFR decreases by ≈1.05 ml/min per year in very old persons. S‐urea seems to be more sensitive than s‐creatinine for indicating renal function in the elderly. The best formula for estimation of clearance is that of Levey. The Cockroft–Gault formula seems to underestimate GFR.