Editor,—I was interested in the two papers on respiratory rate1 2 in the January edition of this journal, because this physical sign and its interpretation fascinate me.

With various caveats Palafox et al suggest that tachypnoea is useful for identifying pneumonia.1 I am concerned that the only children included in their study were those presenting to hospital with acute respiratory infections. To generalise from this setting to the usefulness of this sign outside this specific context could be misleading. In the community a high respiratory rate is quite common in normal healthy infants when awake and interested in their surroundings. The sensitivity and specificity of a diagnostic test depend on the population included. These are likely to have been very different if all children presenting to the hospital or a group seen at random in the community had been included. In our study3 of 1007 infants under 6 months old seen in the community (n = 298) and presenting to hospital (n = 709) there were 30 with a chest x ray picture showing changes of a lower respiratory infection. Their respiratory rate was not significantly different from that of the other 953 children.

Interestingly, in the adjacent paper of Rajesh et al, investigating 200 infants …