Am. J. Respir. Crit. Care Med., Volume 165, Number 8, April 2002, 1084-1092

Sex-Specific Prediction Equations for max_FRC in Infancy
A Multicenter Collaborative Study

Ah-Fong Hoo, Carol Dezateux, John P. Hanrahan, Tim J. Cole, Robert S. Tepper, and Janet Stocks

Portex Anaesthesia, Intensive Therapy and Respiratory Medicine Unit; Centre for Paediatric Epidemiology and Biostatistics, Institute of Child Health and Great Ormond Street Hospital for Children National Health Service Trust, London, UK; Channing Laboratory, Brigham and Women's Hospital, Boston, Massachusetts; and Department of Pediatrics, James Whitcomb Riley Hospital for Children, Indianapolis, Indiana

Measurements of maximal flow at functional residual capacity (max _FRC) from partial forced expiratory maneuvers remain the most popular method for assessing small airway function in infants and young children. However, the lack of appropriate reference data that are both applicable outside the centers that developed them and reflect the normal variability between healthy subjects has limited interpretation of max _FRC results in both clinical practice and research. To address this problem, we collated max _FRC data from 459 healthy infants (226 boys) tested on 654 occasions during the first 20 months of life from three collaborating centers. Multiple linear regression analysis indicated that sex, age, and length were important predictors of max _FRC, which was, on average, 20% higher in girls than in boys during the first 9 months of life. (max _FRC)^0.5 (ml · second¹) = 4.22 + 0.00210 × length² (cm) for boys (RSD = 3.01; R² = 0.48), and 1.23 + 0.242 × length for girls (RSD = 2.72; R² = 0.49). Alternative models incorporating both age and length z scores are also described. Failure to use sex-specific prediction equations for max _FRC may preclude detection of clinically significant changes in girls and lead to false reports of diminished airway function in boys. Appropriate use of z scores, which indicate a "normal" range (z scores of 0 ± 2) for max _FRC, during infancy should also improve interpretation of both clinical and research studies.

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