Oexle et al. (A-7) calculated the negative predictive value of a test for carriers of X-linked ornithine transcarbamylase deficiency (OTCD—a disorder of the urea cycle). A test known as the “allopurinol test” is often used as a screening device of potential carriers whose relatives are OTCD patients.

They cited a study by Brusilow and Horwich (A-8) that estimated the sensitivity of the allopurinol test as .927. Oexle et al. themselves estimated the specificity of the allopurinol test as .997.

Also they estimated the prevalence in the population of individuals with OTCD as 132000. Use this information and Bayes’s theorem to calculate the predictive value negative of the allopurinol screening test.