Although marrow is almost always "clean" in MCAS, I have enjoyed a roughly 90%
"hit rate" on finding aberrant mast cells (either increased quantities and/or
abnormal immunophenotypes and/or genotypes) on random ("blind") biopsies taken
throughout the GI tract. In my patients who want "tissue proof," or for those
patients who will be challenged by their other physicians who won't "believe"
they have MCAS until "tissue proof" is demonstrated, I don't even bother with
marrow biopsies any more and instead just send them to the gastroenterologist
for upper and lower endoscopy. I had to persuade him the first time, but the
gastroenterologist now knows to take multiple biopsies all up and down the
tract. Similarly, I had to persuade him the first time, but the pathologist now
knows to perform special staining (typically at least CD117, sometimes also
Giemsa, tryptase, toluidine blue, CD25, and/or CD2) on *all* the biopsies. The
disease has a patchy and sparse distribution, after all, and the
gastroenterologist is "blindly" taking biopsies of tissues which grossly (to his
eye, through the endoscope) appear completely normal. In fact, without the
special staining, under the microscope the tissue will almost always appear to
the pathologist, too, as either completely normal or, at most, mildly
chronically inflamed. Yet, if the biopsies are taken in a patient who not only
has a history consistent with MCAS but also has at least one elevated mast cell
mediator level, then in at about 90% of these patients you can find at least one
GI biopsy that shows abnormal mast cells -- and I would be willing to bet
another 9% or so would be found to have it if only a few more biopsies had been
taken. Once an abnormal biopsy is found, the pathologist knows to send it for
"PCR" (polymerase chain reaction) analysis for the KIT-D816V mutation that marks
for systemic mastocytosis, but that's almost always negative in MCAS, consistent
with Dr. Molderings' recent findings on the genetics of MCAS. I continue to
hope that we will gain within the near future the ability to (cheaply,
routinely, commercially) analyze for all the mutations Dr. Molderings has found.