It's common knowledge that carpal tunnel syndrome (CTS) is significantly more frequent among patients with diabetes, hypothyroidism, and connective tissue disease (CTD). For this reason, we're all taught to screen CTS patients for these diseases, especially if the condition is bilateral. A recent study in the Journal of Neurology Neurosurgery and Psychiatry asked whether this approach is really justified. Rijk et al. studied 516 CTS patients retrospectively to see if any of the abovementioned diseases was diagnosed by blood testing done at the time of CTS diagnosis.
Of 432 patients with no prior history of diabetes, 10 patients had raised glucose, but subsequent testing confirmed the diagnosis in only 2 patients. Of 468 patients with no history of thyroid disease, 9 patients had an abnormal TSH, but only 2 actual cases were confirmed. In 472 patients with no history of CTD, 40 patients were found to have an elevated ESR but no cases of any CTD were made.
The PPV of CTS for diabetes was .5%, and 216 patients had to be tested for each diagnosis made. For hypothyroidism the PPV was .4%, and 234 patients were tested for each case found.
Overall a rather meagre success rate given that the prevalence of the diseases in the general population is about 4% for diabetes and 9% for hypothyroidism. The false positive rate was rather high as well which ran up the costs because of follow-up testing. The authors put it at about 1400 Euros to make a single diagnosis of hypothyroidism and about 350 Euros per case of diabetes. They didn't do any further calculations on CTD because none were found and because they probably realized how dumb it was in the first place to look for them based on an ESR.