The temptation to order a CT scan is great indeed when faced with a child, a history of head injury, and frightened parents (with or without their lawyers in attendance). But CT scan carries a heavy radiation dose, and radiation induces cancer. For every 1000 to 5000 paediatric CT of the brain, you get one extra brain cancer, and every year about 150 000 children in the USA have a CT for head injury. All of this info comes from a paper which describes the development and validation of a rule intended to allow a doctors safely and accurately to say that a head injury does not warrant a CT.
This is a model of how to develop and validate a classification: hige database, separate derivation and validation samples etc.
The rule had a negative predictive value of 100% and sensitivity of 100% in children under 2 years old (1176/1176 with no clinically important injury and 25/25 with an injury correctly classified). The equivalent figures for a different rules in patients over two were 3798/3800 and 61/63. Of course even in under 2s the performance is not necessarily perfect, with lower confidence intervals on the 100%s being 99.7% and 86% respectively. So there is room for absolutists to argue that you still need a CT to be absolutely certain.
Hopefully this definitive paper will stop lawyers suing when doctors don't take a CT. (But knowing how lawyers behave in the US, we probably need a lawyer to sue a doctor who did order a CT when the child goes on to develop a brain cancer.)
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