3.5.2 In an article entitled "Bucket-handle Meniscal Tears of the Knee: Sensitivity and Specificity of MRI signs, Dorsay and Helms (A-6) performed a retrospective study of 71 knees scanned by MRI. One of the indicators they examined was the absence of the "bow tie

sign" in the MRI as evidence of a bucket-handle or "bucket-handle type" tear of the menis- cus. In the study, surgery confirmed that 43 of the 71 cases were bucket-handle tears. The cases may be cross-classified by "bow tie sign" status and surgical results as follows: 3.5.3 Positive Test (absent bow tie sign) (T) Negative Test (bow tie sign present) (T) Total Tear Surgically Confirmed (D) Tear Surgically Confirmed As Not Present (D) Total 38 10 48 18 23 5 43 28 SOURCE: Theodore A. Dorsay and Clyde A. Helms, "Bucket-handle Meniscal Tears of the Knee: Sensitivity and Specificity of MRI Signs." Skeletal Radiology, 32 (2003), 266-272. 71

(a) What is the sensitivity of testing to see if the absent bow tie sign indicates a meniscal tear?

(b) What is the specificity of testing to see if the absent bow tie sign indicates a meniscal tear?

(e) What additional information would you need to determine the predictive value of the test?

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sign" in the MRI as evidence of a bucket-handle or "bucket-handle type" tear of the menis- cus. In the study, surgery confirmed that 43 of the 71 cases were bucket-handle tears. The cases may be cross-classified by "bow tie sign" status and surgical results as follows: 3.5.3 Positive Test (absent bow tie sign) (T) Negative Test (bow tie sign present) (T) Total Tear Surgically Confirmed (D) Tear Surgically Confirmed As Not Present (D) Total 38 10 48 18 23 5 43 28 SOURCE: Theodore A. Dorsay and Clyde A. Helms, "Bucket-handle Meniscal Tears of the Knee: Sensitivity and Specificity of MRI Signs." Skeletal Radiology, 32 (2003), 266-272. 71 (a) What is the sensitivity of testing to see if the absent bow tie sign indicates a meniscal tear? (b) What is the specificity of testing to see if the absent bow tie sign indicates a meniscal tear? (e) What additional information would you need to determine the predictive value of the test? 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 rel- atives are OTCD patients. They cited a study by Brusilow and Horwich (A-8) that estimated the sensitivity of the allopurinol test as .927. Ocxle et al. themselves estimated the speci- ficity of the allopurinol test as .997. Also they estimated the prevalence in the population of individuals with OTCD as 1/32000. Use this information and Bayes's theorem to calcu- late the predictive value negative of the allopurinol screening test.