The authors thank Serkan Tapan and Erdim Sertoglu for their interest and valuable comments. They stated that in VLBW infants, it would be better to evaluate 25(OH)D levels by LC-MS/MS methods in order to avoid falsely elevated results originating from C-3 epimers. We agree that (LC–MS/MS)-based methods are superior to radioimmunoassay for their high sensitivity and their ability to analyze different isomers and metabolites of vitamin D. However, these methods require sophisticated and costly equipments, which are not always available in clinical laboratories in developing countries. Moreover, in most of the studies 25(OH)D was often measured by immunoassay rather than by mass spectrometry. Other hand, the study showed a high prevalence of vitamin D deficiency in VLBW infants at birth and at hospital discharge. Considering that radioimmunoassay method leads to falsely elevated 25(OH)D levels, the actual prevalence of vitamin D deficiency would have been higher than that observed in the study.
In conclusion, although LC-MS/MS methods could provide more accurate data on 25(OH)D levels, we believe that radioimmunoassay method is suitable for this study and it is unlikely that the conclusions be modified using this method.