Don't Rely Too Heavily On Patch Test Response
Author: Kate Johnson, Contributing Writer
ORLANDO, FLA. -- Don't place too much importance on that weak patch test
result, because it could let you down.
A positive patch test is no more than a clue to help identify possible
environmental hazards to patients, Dr. Robert Rietschel said at the
annual meeting of the American Contact Dermatitis Society.
False positives, false negatives, and lack of uniform reactivity over
time and test site are all obstacles that could render an incorrect
result.
"The significance of a reaction can only be determined over time based
on patient outcomes," said Dr. Rietschel, who is chair of the department
of dermatology at the Ochsner Clinic in New Orleans.
Even reading a patch test, using standard criteria, can be highly
subjective in the hands of experts.
"We know from published data that the experts don't use these rating
systems equivalently," he said. "In one published study, experienced
dermatologists varied from seeing as few as 163 reactions to up to 203
when all circumstances are ideal and they're trying their best to read
very intelligently," he said.
Even morphologic readings can be interpreted differently. For example,
vesicles are not always a criterion for a 2+ response.
"People let situational circumstances override morphology and end up
reading [a patch test] the way they really feel it ought to be read.
That's situational reading and it's not good science."
Dr. Rietschel said the weak patch test response -- which he has
nicknamed a "Janus reaction" -- presents one of the most difficult
interpretation challenges to the dermatologist.
This type of reaction is named for the Greek mythologic figure of Janus,
the double-faced god of gates who looks both backward and forward. This
name was chosen to represent the fact that such reactions may be either
weak allergy or a false-positive result. A typical Janus reaction
"doesn't show overt signs of irritation such as necrosis or epidermal
glazing, has erythema with or without edema, no vesicles, no pustules,
and would otherwise be called anything from a plus-minus to a 1+ or
maybe even a marginal irritant," Dr. Rietschel commented.
"Is it the tip of the allergy iceberg or a false echo, which we should
ignore?" he asked.
Classical teaching would recommend retesting, but this presents another
set of variables, he said.
Studies show that retesting produces the opposite reaction in about
one-third of cases; positives may become negatives, and negatives may
become positives. A patient's level of reactivity does not remain static
over time, nor is it uniform over different sites of the body.
Areas with previous dermatitis react to a greater degree. Areas of more
recent dermatitis have higher reactivity, and the presence of other
positive reactions nearby can affect the result.
The repeat open application test sometimes can prove useful in these
cases, producing unambiguous positive results from weak patch tests in
40% of cases, Dr. Rietschel said.
Instead of placing so much importance on the patch test result,
dermatologists should think more in terms of establishing the relevance
of that result.
"You have to work with the patient and that information to figure out if
their level of reactivity is important. Does avoidance pay off with
improvement? And if not, look elsewhere. This all takes time," he said.
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