If the
interpretation of pheo marker results is already complicated, our current
season adds one more variable for us to consider.
A seasonable
paper on seasonal variations of pheo marker levels was just published. The researchers
charted pheo blood marker levels in each month of the entire year in people without pheo. They found that the
normetanephrine levels were about 20% higher in wintertime than in summertime. The
normetanephrine levels did not vary between seasons in patients with pheo. As you probably have guessed,
the study was done in the Netherlands and Germany (one city in each country)
where there are big temperature differences between winter and summer.
Actually, during the same wintertime, the normetanephrine levels are a little
higher in the colder city. To prove that the cold temperature causes elevation
of normetanephrine levels, the researchers warmed the forearms where blood was
drawn and found the levels dropped.
The 20% higher blood
normetanephrine levels in wintertime do not sound too much but can make a
result from high normal to borderline abnormal. We already know that anxiety, stress,
sleep apnea, hypertension, older age, and some medications are associated with
higher normetanephrine levels. All these factors, including wintertime, need to
be weighed before a decision is made about the lab results. As I have
reiterated many times before, the pheo diagnosis should not be based on a
single lab test but should be based on the totality of the patient’s condition.
Clinical presentation, lab results, and imaging characteristics are all
important.
It would be
interesting to do the same study in a tropical area like Hawaii. We would
predict that the blood normetanephrine levels in people without pheo remain
similar throughout the year.
The new
wintertime confounding factor in pheo test result interpretation reminds us once
again that a test result is influenced by many factors. Simply calling a test
result positive or negative will result in misdiangosis.
Dr. Pheo
