Allergists like to read skin tests.  And we think we're pretty good at it.  And we are--for the first few minutes...But there's a catch:  after reading the initial test response at about 15 minutes, we think the job is over.  And premature judgement is sometimes worse than no judgement at all.  And it doesn't matter whether we're the President and we stride across an aircraft carrier and announce "Mission Accomplished" to the world , or whether we allergists stride across the room and announce "Mission Accomplished" to our patients after the initial prick test reading, in reality the job isn't done.  In fact, we're in a big mess.  I'm reminded of the joke:  What does the allergist like to watch on TV?  Answer:  Anything at all--but only for the first 15-20 minutes...

In truth, after we're done watching "Prime Time" (the first few minutes of the prick test) we "turn off the TV".  And that's a shame, because The Late Show is just beginning.  And, in my opinion, it's a far more interesting show than anything running on "Prime Time".  It stars (none other than) The Delayed Skin Test Response, otherwise known as the Late-Phase Cutaneous Reaction (LPCR for short).  I've been watching this "late show" now for 27 years (30 if you include my fellowship), and it never ceases to intrigue, surprise, and confound me.  David Letterman couldn't do it better.  I said it before, and I'll say it again:  As allergists, we should be following the skin test to extinction.  This series will be based around the following 3 areas: 

•  Part I:  Histopathology, Immunology & characteristics of the LPCR
• Part II:  Diagnostic pearls for the LPCR:  clinical correlations
• Part III: Treatment pearls for the LPCR

But first, before beginning, in the spirit of The Late Show, I've got a "Top Ten" list for you.  So, Here goes:  The Top Ten Reasons to Recognize and Treat the LPCR:

Reason #10:  It can explain puzzling reactions to traditional "build-up" immunotherapy that sometimes occur in patients

Reason #9:   It can help explain many puzzling aspects of your patients history that otherwise couldn't be explained

Reason #8:   It can explain--and help you relieve-- alot of debilitating symptoms your patients have

Reason #7:   LPCR's occur in lots of patients.  I mean lots.

Reason #6:   There is an effective treatment for LPCR's in patients:  SLIT

Reason #5:   Looking for and treating LPCR's will make you look smart to your patients 

Reason #4:   If you can help these patients, they'll bring in more patients to your office for treatment--often the "bread-and-butter" atopics that your heart lusts after (a la Jimmy Carter) 

Reason #3:  If you understand the implications of the LPCR, you won't have to read this stupid list anymore.

Reason #2:  You'll want to tune into the Late Show LCPR rather than just watching the Prime Time Prick (test, that is!) 

Reason #1:  You'll be smarter than David Letterman. 

Now if that list  doesn't put some propellant into that "demo inhaler" on your desk, I don't know what will.  But before we get "to the good clinical stuff", let me review some basics about the LPCR which we all should be familiar with.  Please yawn now and get it overwith, because here it comes:

• When you look under the hood, you'll see the LPCR is really a mixed cellular infiltrate with edema, and sometimes fibrin deposition.  There is no vascular damage, IgG, IgA, or IgM deposition.  Nor is their evidence of complement activation.  A variety of cells, including macrophages, eosinophils, tryptase positive mast cells, and neutrophils can be found. 
• The LPCR is in part mediated by antigen-specific MHC restricted T-cells.  T cells are present in the inflammatory LPCR infiltrate, and the LPCR is thought to be partially dependent on them.
• Documented agents stimulating immunologic activation of the mast cells that have induced the LPCR include: molds, pollens, danders, mites, and enzymes, some drugs, anti-IgE antibodies, and possibly some foods
• There are at least 4 randomized, controlled clinical trials of immunotherapy in patients with immediate skin test reactivity & LPCR's who had reduction in LPCR's and symptoms with immunotherapy.  However, to my knowledge, there have been no clinical trials of immunotherapy on patients with no immediate skin test reactivity and only LPCR's--and as you'll see later, this is where SLIT excels.  Like gangbusters. 

   

  Stop yawning now.  As I've mentioned above, I've been observing LPCR's in my allergy patients for a long time; I've been observing these LPCR's while employing IDT (or SET) testing--not prick testing--in my practice, and my observations are thus within this context.  (Why I prefer IDT over standard prick testing is a subject worthy of a separate entry in itself, but we'll let that pass for the moment).  Anyway, although I've never seen it directly proposed, It seems that there are two distinctly different type of LPCR's I have seen in practice: 

Type 1:  A LPCR following a skin test that is positive for immediate reactivity.  These are the reactions that occur after a patient typically has an initial strong response to something such as ragweed or dust.  They have a strong immediate AND delayed reaction.  In general, these LPCR's occur fairly early, typically within a few hours of initial application, and swell alot during the next 6-12 hours, when they should be read when the response plateaus, and are usually gone in 24-48 hours.  They don't usually blister or ulcerate, and leave no residual mark  at the site.  

Type II:  A LPCR following a skin test that is negative for immediate reactivity.  These occur in the patient who has "disappointing" skin tests.  You know, the farmer who swears he has sinus problems in the spring and fall, and his initial 10 minute reading shows virtually nothing.  Typically, these LPCR's have a later onset--typically 12-24 hours after application, peak at 24-48 hours, and can linger for days, and in extreme cases can blister and even leave a persistent mark where they were applied.  These are the Energizer-Bunny reactions--they just keep going and going and going... Molds, in particular, are classic to give this response.  These reactions should be read at 24 and at 48 hours after application.   These can be particularly nasty.

I can almost hear you asking at this point:  "So we see some patients exhibit a LPCR--so what?; we don't know what the LPCR really means".  Well, Jerome, you don't know what it means because you're not looking.  I've been looking for 30 years, and the next part of this series will talk about what a patient with LPCR "looks like" from a clinical perspective.  What the LPCR really means from the patient's point of view.  So stay tuned.  The plot thickens.  Speaking of which, in view of the time course of the LPCR maybe I should have named this series "24 hours" instead of "The Late Show"--or is that title for a series already taken?