Short-term measurement of linear growth in preterm infants: validation of a hand-held knemometer

A hand-held electronic knemometer, resembling a pair of callipers, for measuring knee-heel length in preterm infants was developed to improve the accuracy of measuring linear growth velocity in infants. The measuring system is based on a magnetic encoder and has a resolution of 0.01 mm. The knee-heel length is recorded automatically when the pressure applied on the heel reaches a preset value. The result of a measurement sequence is expressed as the average of five sequential readings. The error of one measurement sequence was 0.82 mm, corresponding to a coefficient of variation of 0.8% or 2 d of growth in knee-heel length. It includes the technical error and an error component due to the correlation of the readings within a series. The estimated error on the measurement of the knee-heel growth velocity (expressed as mm/d), measured over a 3-wk period, was 0.04 mm, corresponding to a coefficient of variation of 8%. Longitudinal growth data from 11 healthy preterm infants (birth weight 918-1482 g) are presented. Knee-heel length velocity from birth until day of regained birth weight was similar to the velocity during the following weeks, showing that there was no deceleration of growth immediately after birth. In conclusion, the method is accurate and gentle in measuring linear growth velocity over short periods and can be useful in monitoring the progress of ill infants and in growth studies of preterm and mature infants, in which linear growth is a more relevant short-term outcome than weight gain.