Juillard L, Fouque D, Hadj-Aissa A, Laville M, Maakel N,
Pozet N
Is it worth indexing GFR to body surface area?
Am Soc Nephrol
J Am Soc Nephrol (abstract)
(Sep) 7:1582 1996
Physiologic parameters such as GFR are often indexed to body
surface area (BSA), i.e. the parameter is divided by BSA and
multiplied by 1.73, the idealized BSA. This is based on the
presumption that many physiologic functions are proportional to BSA
and that indexing will remove variations based on size and weight, and
clarify true variations in physiologic function.
The study of Juillard et al. is clearly a response to the intriguing
and provocative study from another group, Turner et al.(1) The latter
assert that indexing of variables such as GFR for BSA is "devoid of
statistical foundation" and may at times lead to the appearance of
spurious correlations, and in other circumstances may obscure the
presence of true correlations. It was proposed that indexing of a
variable for BSA would be appropriate only when a linear regression
plot of the variable against BSA revealed a significant positive
correlation, and a Y- intercept of zero. In addition, once indexed,
the indexed variable should have no residual correlation with BSA.
The current study by Juillard et al. examined whether GFR
satisfied these criteria. GFR was measured by inulin clearance in 396
normal adults and children. A strong positive correlation was found
between GFR and BSA, which had a Y-intercept not significantly
different from zero. When GFR was indexed for BSA this
correlation was completely lost. They concluded that GFR satisfied
the conditions for indexing.
Comment: Turner et al. [1] offer a persuasive warning against
an overly credulous acceptance of indexing as a means of negating the
effect of BSA, height or weight, etc. Unfortunately the alternative
they offer, regression, is moderately complex, especially as compared
to indexing. Fortunately Juillard convincingly demonstrates a sound
statistical basis for indexing GFR for BSA.
Reference:
1) Turner S, Reilly S: Am J Physiol 268:R978-R988:1995.
(Greg Cowell, M.D., University of Illinois at Chicago)
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Am Soc Nephrol
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