Some ailments by their nature are difficult to diagnose, for example diagnoses of exclusion, and new strains of re-emerging diseases. Idiopathic anaphylaxis is a diagnosis of exclusion; a physician must eliminate all other known causes of anaphylactic attack before labelling it idiopathic.
Re-emerging diseases often differ somewhat in the symptoms and vectors from the classical forms of the disease, and the drugs that treated the standard strain of the disease may not work on the new form. This makes it hard to identify an epidemic that is brewing.
A WHO 1993 survey of 34 laboratories worldwide that were supposed to give early warning of epidemic disease revealed that only half the laboratories could diagnose yellow fever reliably, 56% couldn't identify hantaviruses, 82% didn't diagnose California encephalitis, and most laboratories were not equipped at all to test for rarer viral infections, such as Ebola, Lassa and Machupo.
The diagnosis of pulmonary embolism (PE) is missed more than 400,000 times in the USA each year, and approximately 100,000 patients die who would have survived with the proper diagnosis and treatment. It is especially likely to be missed in older patients. The correct diagnosis of PE is made in 30% of all patients who die with massive PE but in only 10% of those who are 70 years of age or older. It is the most commonly missed diagnosis responsible for death in the elderly institutionalized patient. Untreated, approximately one-third of patients who survive an initial pulmonary embolism will die from a future embolic episode.