The measurement of knee‐heel length in newborn infants using a simple vernier calipers

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Abstract
Objective: To assess the usefulness of a simple vernier calipers for measuring knee‐heel length in neonates. Subjects and methods: Using a simple vernier calipers, knee‐heel length was measured five times by 2 observers in 50 babies (29M, 2IF; mean birthweight 1597 g; median gestational age at birth 29 weeks) at a median postnatal age of 11 days. A subgroup of 20 babies had knee‐heel length measured similarly at weekly intervals for 3 weeks. Corrected gestational age and weight were simultaneously recorded. One observer was experienced in using the vernier calipers. The precision of the calipers was established using 4 steel gauge blocks of varying length (7.62‐10.17 cm). Results: The calipers were very precise when measuring steel gauge blocks. In babies, there was a downward trend across the first 2 measurements for both observers, the measurements stabilizing over the last three. Using the final three measurements per baby (n= 50), the experienced observer had a mean standard deviation of 0.023 cm and mean coefficient of variation 0.23%‐ when measuring an average knee‐heel length of 9.99 cm. The inexperienced observer had a mean standard deviation of 0.057 cm and a mean coefficient of variation of 0.56%, when measuring an average knee‐heel length of 10.14 cm. The inter‐observer reliability, measured by the intra‐class correlation coefficient, was 0.99. The agreement between observers was such that one observer measured knee‐heel length consistently less (0.15 cm. SD 0.18 cm) than the other. The reliability for knee‐heel length velocity was lower (R= 0.85), but agreement between observers was high with an average difference of 0.016 cm/ week. Knee‐heel length was significantly correlated (p < 0.001) with corrected gestational age (r= 0.85) and with weight (r= 0.96). There was a weaker but significant correlation (r= 0.47, p < 0.001) between knee‐heel length velocity and rate of weight gain (g/day), indicating that weight gain may not always be accompanied by an increase in linear growth. Conclusion: The measurement of knee‐heel length by a simple vernier calipers is an accurate, reproducible and non‐invasive method of assessing short‐term linear growth in neonates. However, it is recommended that measurements of knee‐heel length in a individual baby should be made by a single experienced observer.