To the Editors:
We are interested to read the letter by Day suggesting that Song's formula was more reliable in children between the ages of 4 months and 13 years compared with Strobel's or Jolley's formulas.
We have reviewed our data for the premature neonate between 28 and 35 weeks gestation and find that Song's formula for calculating esophageal length in our preterm population of infants would result in an estimated esophageal length that was on average 9.75 cm (range 8.6–13 cm). The use of Strobel's formula for calculation of LOS position was, on average, 14.92 (range 13.5–18.3 cm). Comparison with Strobel's formula using the recommended 87% of the LOS calculation with correct probe position on radiograph yields an average re-positioning of 0.88 cm (range 0.3–2.3). In the infants in whom probe repositioning was indicated on radiograph, it was necessary to withdraw the probe slightly, except in two infants when it was passed marginally further down the esophagus. If Song's formula for estimation of esophageal length were used for probe placement, the difference between probe position on radiograph and Song's calculation would be on average 2.9 cm (range 1.10–4.4 cm) and in all cases the probe would be positioned too high in the esophagus.
Therefore, citing the probe based on Song's calculation, particularly in the population of very small preterm infants studied, could potentially result in significant errors in interpretation, since the probe would be around the middle of the esophagus rather than just above the LOS. In view of these comparisons, it was believed that Strobel's formula was the non-invasive method of choice for probe placement in the preterm neonate.