We read with interest the paper by Bertenshaw et al1 on the incidence of acute renal failure (ARF) in patients with cystic fibrosis (CF) and would like to supplement these results with our findings. With increasing survival and therefore consideration for lung transplantation, monitoring adult patients with CF for renal impairment is assuming increasing importance (creatinine clearance <50 mg/ml/min is a contraindication to lung transplantation).2

We examined the prevalence of renal impairment in our stable adult CF population3 in Birmingham using the Cockcroft-Gault formula4 for estimating glomerular filtration rate (GFR). Between 1 January 2004 and 9 February 2005, 207 of our 273 adult patients with CF had at least one serum creatinine measurement; eight were excluded from analysis owing to established renal failure, one had subnormal creatinine precluding analysis and in six cases the data were incomplete, leaving 192 patients for analysis. Using the Cockcroft-Gault formula and the United Kingdom National Kidney Foundation definition of renal impairment (GFR <90 ml/min), 50 of the 192 patients analysed had renal impairment; 26 of these had a current infective exacerbation of which 12 were receiving intravenous antibiotics. Thus, excluding those in a current exacerbation and those with established renal failure, 24/192 (12.5%) of our patients with stable CF had renal impairment. This is probably a conservative estimate as we know from a study in Liverpool in a Pseudomonas-colonised CF population that 24-hour urine collection diagnoses more renal impairment than the Cockcroft-Gault formula.5 Only one of the 50 patients identified as having renal impairment by the Cockcroft-Gault formula had a raised serum creatinine level, which is often the only test that first draws our attention to the problem.

Our findings support the need for regular renal assessment in adult patients with CF using measures superior to raised serum creatinine levels alone (eg, at least using the Cockcroft-Gault GFR formula for all routine annual reviews) to enable early diagnosis while we await “firmer inferences to be drawn regarding the causation of ARF in CF and allow avoidable precipitating factors to be identified”.