Monday, December 7, 2009

Interferences with pheo tests

The 5 pheo tests are subjected to interferences by quite a few factors. I list those factors here according to their significance. The most important message, however, is that these factors are usually not a big deal and can be rather easily figured out by an expert. Reaching a pheo diagnosis requires all things considered, rather than any evidence in isolation.

Major interferences:

1. In many diseases, catecholamines and metanephrines levels are indeed genuinely elevated, and can be rather high. About 30 such diseases are known. Obstructive sleep apnea, severe anxiety, and essential hypertension are the most common ones.

2. Any major stress, such as stroke, severe infection, and bad pain, elevate catecholamines and metanephrines levels.

3. Some drugs such as cocaine, phenoxybenzamine, tricyclic antidepressants, and monoamine oxidase inhibitors can also elevate catecholamines and metanephrines levels. Sinemet elevates dopamine levels tremendously.

Minor interferences:

Eating, standing, the stress of venipuncture, and old age. These conditions usually elevate catecholamines and metanephrines levels only slightly.

Variable interferences:

These are caused by drugs (usually beta blockers) and they are assay-dependent. The clinicians and the lab should communicate about them.

Errors:

Clerical and lab errors are also realistic possibilities. They are rare but can happen.

When I order pheo tests, I actually tell the patients initially not to worry about any interferences. If the results are normal (which is the case in most patients), the patient does not have pheo. If the results are elevated (usually in 20% of cases), I will then decide on a case-by-case basis.

This will be the last post this year. Happy holidays! I will continue posting next year.

Dr. Pheo