This study shows that LBW newborns in Uganda can be identified using anthropometric surrogate measures on the first day of life. In resource poor settings where neonatal mortality remains high and many births occur at home, the missed opportunities for either providing life saving care at home or referral are mainly a result of a failure to identify high risk LBW newborns. Anthropometric measurements have been found to be reliable in identifying LBW. However, studies show that the measure of choice and its cut-off point is dependent on the context [10, 14–16], therefore the need to conduct area specific studies. Previous studies show that more work has been done for other anthropometric measurements like HC, CC, TC and MUAC than for foot length as a predictor of LBW. This is one of the few studies that compares FL with other measurements.
Although in this study all the measurements could identify LBW to some degree, FL had a fairly strong correlation with birth weight (r = 0.76 and AUC = 0.97) although from the value of r2, only 58% of the variation in foot length can be explained by the linear relationship between FL and birth weight. Other analysis (sensitivity, specificity, +LR, -LR and DOR) indicate that the two most appropriate measurements for identifying LBW in Uganda are FL and CC.
A foot length cut-off of 7.9cm has a sensitivity of 94% and specificity of 83% for predicting LBW. At a cut-off of 7.9 cm the probability of a baby being low birth weight increases from 12% to 66% when the foot length is less than 7.9 cm (Table 3). Using a more practical cut-off of 8.0 cm for foot length increases the post-test probability from 12% to 64%.
The operational cut-off determined by our study for the different measures are comparable to cut-offs obtained in similar studies in some settings. For instance the cut-off for CC was 31.0 cm in this study which is comparable to the cut-off in Nepal (30.8 cm) and in Iran (31.2 cm) [8, 9]; and that for HC was 33.3 cm in comparison to 33.5 cm in Nepal . In this study foot length cut-off of 7.9 cm is comparable to 8.0 cm that was proposed by Marchant et al. in Tanzania .
On observation during the data collection, the midwives were finding it difficult to determine the exact point to measure the thigh, head and mid upper arm circumferences. Also timing the end of expiration to measure the chest circumference was challenging. This could have implications for use of these measurements by community health workers. However, foot length was practically easier and faster to measure. The ease with which foot length was measured compared to the rest makes it a preferred measure. A problem of accuracy of HC measurements due to moulding of the head especially when labour was prolonged or obstructed was noted by Dhar et al. . Another advantage is that measuring the foot does not require undressing the baby thus exposing them to cold.
Although in this study, we did not test the usefulness of these measures after day 1 in identifying LBW, Marchant et al. demonstrated that foot length was a good predictor of LBW up to day 5 after birth . This would be critical because in some cases the CHWs do not visit the newborns on the first day. A study in Uganda also showed that HC and CC can be measured in the first 2 weeks of life and extrapolated to estimate the measurement at day of birth .
Uganda has launched the Village Health Team concept which is compatible with the WHO and UNICEF recommendation to provide extra care for newborns at home . It involves community health workers visiting homes to promote health, but provides no access to weighing scales in part because of cost and fears about their maintenance and sustainability. A low tech, low cost and low risk tool to identify small babies could support community efforts to save newborn lives if it is added to the VHT resources used during postpartum visits.
In using such a tool, we would be more concerned about minimising numbers of false negatives than false positives. This study tested HC, FL, MUAC, TC and CC, and showed that this is well achieved by FL. The potential risk of including babies that are falsely labelled LBW is minimal compared to the consequences of leaving out truly LBW babies.
This being a hospital-based study, the estimates of LBW may not reflect what is in the community. The measurements were done by health workers but the tool will be used by community volunteers and their skills are likely to be different. However a validation of this tool will be done in the community with community volunteers. There was potential for bias by having the same midwife perform both gold standard and index measurements, but this was minimised by the investigator not knowing the cut-off points for the index measurements. The exclusion of very small babies could have led to under estimation of the true sensitivity and specificity since these are more likely to be observed by the proxy measurement.