Comparison of the use of body mass index percentiles and percentage of ideal body weight to screen for malnutrition in children with cystic fibrosis.

BACKGROUND The Cystic Fibrosis Foundation (CFF) recommends using the percentage of ideal body weight (%IBW(CFF)) and body mass index percentiles (BMIp) to assess weight-for-height status and to screen for malnutrition. OBJECTIVE The objective was to examine the agreement and discrepancy between the use of %IBW(CFF) and BMIp for screening malnutrition. DESIGN Data from 13 021 children reported to the 2000 CFF Patient Registry were analyzed. RESULTS In children of average stature (ie, height-for-age between the 25th and 75th percentiles) and aged <10 y, %IBW(CFF) corresponded closely to BMIp, and the prevalence of underweight estimated by %IBW(CFF) < 90% was similar to that by BMIp < 15th percentile. However, in children with short stature (ie, height-for-age < 25th percentile), %IBW(CFF) reflected significantly better weight-for-height status than did the BMIp, whereas the opposite trend was observed in children with tall stature (ie, height-for-age > 75th percentile). Such discrepancies averaged 8-12 percentage points when BMIp was reexpressed to the same unit and scale as %IBW(CFF). Consequently, the prevalence of underweight estimated by %IBW(CFF) < 90% was significantly lower (7.3%) than that estimated by BMIp < 15th percentile (25.7%) in children with short stature, whereas the opposite trend was found in children with tall stature (47.7% and 14.4%, respectively). Additional analyses showed that BMIp was more sensitive to, and had stronger associations with, the percentage of predicted forced expiratory volume in 1 s. CONCLUSION Compared with BMIp, %IBW(CFF) underestimated the severity of malnutrition in children with short stature and overestimated the severity of malnutrition in children with tall stature.