How to find the gold standard in each USMLE question.

A 68 year old male presents with 5 days of fever, non-productive cough, confusion, and a chest x-ray showing non-specific pneumonia. He works in a community hospital with a desk underneath an air-conditioning vent. A sputum culture is taken and it grows a large number of organisms on blood agar. What is the most likely cause of his pneumonia?

The answer to this question could be a number of things, but the one thing I'm certain about, is that the answer is NOT Legionella pneumophilia. True, he has exposure to air-conditioning and presents with the typical symptoms of Legionella pneumonia, but when did Legionella ever grow on normal blood agar? It is a fastidious organism that grows on charcoal yeast extract (CYE) agar. Most of the people I ask know the culture requirements of Legionella, but those same people still answer Legionella for the preceding stem. Why does this happen? And how do you prevent yourself from making the same mistake again?

The way I avoid making this mistake on a multiple choice test is by finding the "gold standard" information in each question. Usually the gold standard information is found in the pathology or microbiology report, followed by blood tests, radiology reports, physical exam, patient history, and finally by risk factors, in that order. A good example of this is found in the treatment of cancer. Would you be willing to give a patient months of chemotherapy based on a just a history of smoking and a cough? How about a single calcified nodule in the lungs? How about a pathology report showing small cell lung carcinoma? I think most of us would rely on the later for our treatment decisions.

The same logic can be applied to answering our original scenario. Would you give this patient the drug of choice for Legionella, a macrolide, based on the history of exposure to air conditioners? If that is how medicine was practiced, I would be on erythromycin right now. The differential diagnosis of diseases in people who were exposed to air conditioners includes just about ALL diseases.

Here is my "gold standard" list from most reliable to least reliable.

1. Pathology/histology reports
2. Microbiology cultures/stains
3. Blood tests including CBC, electrolytes, tumor markers, hormone levels
4. Radiology reports
5. Vitals and physical exam
6. Symptoms
7. Patient's past medical history
8. Patient's age
   
9. Patients risk factors
   
10. Family history