BNF calculator for estimating creatinine clearance is unsafe in children.

7 April 2006

It is tempting to calculate the glomerular filtration rate (GFR) in children, in whom the accurate measurement of GFR is challenging. The BNF for children fails to spell out the hazards of estimations based on the Schwartz formula. 1 Renal impairment is overlooked, where serum creatinine is not raised.2 In addition, the BNF Extra offers a serum creatinine, age and sex-based calculator on-line without comment on its use in children.3

We compared creatinine clearance estimated by Schwartz formula and BNF Extra calculator from contemporaneous serum creatinine to laboratory GFR (by single injection 2 sample 99Tc-DTPA methodology) in 114 paediatric urology patients aged 1-14 years (Figure 1).

Figure 1.
Frequency distribution of measured GFR (light grey), Schwartz estimate (dark grey) and BNF on-line calculator (black) in 114 paediatric urology patients aged 1-14 years. The Schwartz estimate produced an absolute error of 19.4% +/-65.2% as compared against the measured GFR. The BNF on-line calculator compared against the measured GFR resulted in an absolute error of 84.3% +/-114%.

The Schwartz formula identified patients with a laboratory GFR <_80 ml="ml" min="min" _1.73m2="_1.73m2" with="with" _91="_91" specificity="specificity" but="but" only="only" _60="_60" sensitivity.="sensitivity." schwartz="schwartz" missed="missed" those="those" in="in" whom="whom" the="the" remaining="remaining" number="number" of="of" functioning="functioning" nephrons="nephrons" was="was" sufficient="sufficient" for="for" a="a" normal="normal" serum="serum" creatinine.="creatinine." as="as" this="this" group="group" patients="patients" nevertheless="nevertheless" requires="requires" special="special" attention="attention" including="including" use="use" medicines="medicines" formula="formula" is="is" unsafe.="unsafe." p="p"/> The on-line BNF calculator overestimated creatinine clearance in every case (Figure 2): mean overestimate 82 ml/min/1.73m2, range 4.5 to 156 ml/min/1.73m2. It should therefore never be used in children.

Figure 2.
Bland/Altman plot of the difference between the BNF on-line calculator and measured GFR against the mean of the measured GFR and BNF on-line calculator in 114 paediatric urology patients aged 1-14 years. The on-line BNF calculator overestimated creatinine clearance by a mean 82 ml/min/1.73m2, range 4.5 to 156 ml/min/1.73m2.

In reply to Dr Marcovitch, who comments that the BNF for children has been validated against emerging evidence, guidelines on best practice and advice from a network of clinical experts, we would caution the use of either of these mechanisms for estimations of GFR in children.4 We suggest that the BNF Extra on-line calculator should exclude its use in children.

Nas Al-Jafari, F2 Trainee in Paediatric Surgery, Naima Smeulders, Specialist Registrar in Paediatric Surgery and Rowena Hitchcock, Consultant Paediatric Surgeon and Urologist. Department of Paediatric Surgery and Urology, John Radcliffe Hospital NHS Trust, Oxford, OX3 9DU. naima.smeulders@btopenworld.com

References:

1 Costello, I ed. BNF for children. London: Pharmaceutical Press, 2005:12-13. 2 Seikaly MG, Browne R, Bajaj G, Arant BS Jr. Limitations to body length/serum creatinine ratio as an estimate of glomerular filtration in children. Pediatr Nephrol1996; 10:709-11. 3 BNF Extra. Calculators: Creatinine clearance. 2005 http://www.bnf.org/bnf/extra/current/noframes/450019.htm (accessed 17 March 2006) 4 Marcovitch H. Safer prescribing for children. BMJ 2005;331:646-47.

FIGURES 1 and 2 emailed to Sharon Davies

Competing interests: None declared

Competing interests: None declared

Naima Smeulders, SpR Paediatric Surgery

Nas Al-Jafari and Rowena Hitchcock

Department of Paediatric Surgery and Urology, John Radcliffe Hospital NHS Trust, Oxford, OX3 9DU.

Click to like: