I read with interest the article by Papacostas et al describing the role of sixth-hour transcutaneous bilirubin (TcB) in identifying infants at low and high risk of phototherapy (PTX) in the first 24 hours. They reported a cutoff value on 3.0 to 5.3 mg/dL as the lower and higher risk for PTX for a subset of infants with ABO incompatibility and positive direct antiglobulin test. Using the likelihood ratio nomogram, as described by Jaeschke et al, the posttest probability for the lower risk TcB of 3 mg/dL would be 37% (the pretest probability 15%, likelihood ratio was 3.7), whereas the posttest probability for the upper-risk TcB of 5.3 mg/dL would be 97% (the pretest probability 70%; likelihood ratio was 20.9). The higher probability noted with higher bilirubin level was expected. In fact, a bilirubin of 5 mg/dL at 6 hours of life, for infants with ABO incompatibility, is...

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