What it is:This appendix is a procedure manual, not a disease. It covers two related tools for a patient carrying a "penicillin allergy" or similar label: skin testing, which objectively confirms or rules out a current IgE-mediated allergy, and induction of drug tolerance protocols(desensitization), which let a patient temporarily receive a drug they're truly allergic to when there's no safe alternative.
The core problem:A reported allergy label often isn't checked against reality. Patients get pushed onto broader-spectrum, less effective, or more toxic alternatives for years because nobody ever confirmed whether the original reaction was actually IgE-mediated, or whether it's even still present. Skin testing answers "is this allergy real and current." Desensitization answers "the allergy is real, but I have no other option, how do I get this drug in safely anyway."
What you do about it:Test before you assume. Negative skin test at every step means proceed toward giving the drug (and confirm with an oral challenge). Any positive step means stop, the patient does not get that drug through the normal route. If the drug is still needed anyway, or the reaction mechanism isn't a classic immediate one to begin with, a monitored tolerance induction protocol is the fallback.
Keep these two ideas separate, they get blended constantly. Skin testing is diagnostic: it tells you whether an IgE-mediated allergy is present right now. Desensitization is a treatment workaround: it assumes the allergy is real and gets the drug in anyway, temporarily, under tightly controlled conditions. A negative skin test means you don't need desensitization. A positive one, or an allergy that can't be skin tested at all, is what pushes you toward it.
Not every drug tolerance protocol looks the same, and the reason is mechanism. The underlying immunologic (or non-immunologic) process behind the original reaction determines how fast you can safely escalate the dose, and how much is actually known about what the protocol is doing.
| Underlying mechanism | Initial dose | Duration | Potential outcome | Example |
|---|---|---|---|---|
| Immunologic IgE(true desensitization) | Micrograms | Hours | Desensitization; renders mast cells less responsive to degranulation | Anaphylaxis to β-lactam antibiotics; taxanes |
| Immunologic non-IgE | Milligrams | Hours to days (roughly 6 hours to 10 days) | Not known | Delayed cutaneous reactions to trimethoprim-sulfamethoxazole |
| Pharmacologic | Milligrams | Hours to days (roughly 2 to 5 days) | Cautious induction of a reaction followed by a shift in a metabolic process | NSAID-exacerbated respiratory disease |
| Undefined | Micrograms to milligrams | Prolonged; days to weeks | Not known | Isolated cutaneous reactions to allopurinol |
Only the IgE-mediatedrow has a defined mechanism and the word "desensitization" attached to it, because that's the one situation where the biology is understood: you're dosing under the threshold that triggers mast cell degranulation, in tiny microgram steps, over a matter of hours, so the mast cells become progressively less reactive instead of firing. The other three mechanisms move slower and cautiously, over days to weeks, precisely becausethe process isn't well characterized. Slower isn't a stylistic choice there, it's what you do when you don't know exactly what you're managing.
The IgE protocol's whole logic is to move fast enough to finish before mast cells "reset," but slow enough at each step that you never deliver a dose big enough to trigger degranulation. That's the tightrope every rapid IV desensitization schedule is walking, including the cephalosporin protocol below.
Prick testing always comes first because it's the less invasive, lower-risk step. You only move on to intradermal testing if the prick test is negative.
Setup:Clean the volar surface of a forearm with an alcohol swab, then mark separate testing sites along the arm with an ink pen. Four solutions get their own premarked spot: Pre-Pen(the major penicillin determinant), diluted Penicillin G(the minor determinant mix), a histamine positive control, and a saline negative control.
Site order matters:working up the arm from the elbow, the order is histamine (most distal), then saline, then Pre-Pen, then Penicillin G.
Technique:apply a small drop of each solution to its own site, then puncture the epidermis at each drop with a twisting motion using a sterile 22-28 gauge needle. Very little pressure is needed, and you should not draw blood.
Read the test at 15-20 minutes.
| Result | What you're looking at | What it means |
|---|---|---|
| Negative | Wheal diameter change is less than 3 mmcompared with the negative (saline) control | Proceed to intradermal testing |
| Positive | Wheal diameter change is greater than 3 mmcompared with the negative control | Wipe the solution off immediately. Do notproceed to intradermal testing |
The positive control (histamine)must actually produce a reaction, otherwise the whole test could be falsely negative and you can't trust a "negative" result. The negative control (saline)must stay negative. If a wheal greater than 2-3 mm develops at the saline site after 20 minutes, repeat the prick test. If the saline site is still reactive on retest, stop testing entirely and notify the ID physician and/or the ID stewardship pharmacist rather than pushing forward with an uninterpretable test.
This step only happens if the prick test was negative. A positive prick test is itself already a stopping point.
Setup:select 5 sites on the volar forearm, ideally on the arm opposite the one used for the prick test.
Technique:using a 26-30 gauge, short bevel needle, intradermally inject 0.02 mL of Pre-Pentwice, at least 2 cm apart. Using separate needles and syringes for each solution, also inject 0.02 mL of diluted Penicillin G(equal to 200 units of penicillin) twice, at least 2 cm apart, and 0.02 mL of saline, at least 5 cm from the other sites. Mark the margins of the initial blebs with an ink pen so you have a true baseline to compare against.
Read the test at 20 minutes.
| Result | What you're looking at | What it means |
|---|---|---|
| Negative | No increase in the original bleb size, and no greater reaction than the negative control site | Patient has a negative skin test overall; an oral challenge may follow |
| Positive | Bleb or wheal increases more than 2 mmfrom its original size, or is more than 2 mm largerthan the negative control | Patient is notto receive penicillin |
Notice the cutoff shrinks between steps: 3 mmfor the prick test, but only 2 mmfor the intradermal test. The intradermal test is the more sensitive of the two (it's only reached after a negative prick result), so it takes a smaller change to call it positive.
This step is optional and only happens if the ordering physician decides it's necessary, typically after a fully negative skin test, to confirm the patient genuinely tolerates the drug clinically and not just on the skin.
How it's done:an oral penicillin (for example, amoxicillin 250 mg) challenge, or a graded challenge of the actual target drug, given in a monitored setting for 30-45 minutes.
Put the three steps together and the logic is a simple funnel: each step only happens after a negative result on the step before it, and a positive result at any point ends the pathway right there.
| Step | Read at | Positive cutoff | If positive |
|---|---|---|---|
| 1. Prick test | 15-20 min | Wheal >3 mm vs. negative control | Stop. Wipe solution off. Do not proceed to intradermal testing |
| 2. Intradermal test | 20 min | Bleb/wheal >2 mm increase, or >2 mm vs. negative control | Stop. Patient does not receive penicillin |
| 3. Oral challenge (optional) | 30-45 min, monitored setting | Not specified in this table; clinical reaction during observation | Confirms whether clinical tolerance matches the negative skin test |
This is a concrete example of the "Immunologic IgE" row from the mechanisms table put into practice: an induction of drug tolerance protocolthat builds a patient up to a full 1000 mg IV cephalosporin dose through a series of small, closely spaced steps.
| Solution | Volume of diluent (e.g., 0.9% NSS) | Total drug in bottle | Final concentration |
|---|---|---|---|
| Solution 1 | 250 mL | 10 mg | 0.04 mg/mL |
| Solution 2 | 250 mL | 100 mg | 0.4 mg/mL |
| Solution 3 | 250 mL | 1000 mg | 4 mg/mL |
| Step | Solution | Rate (mL/h) | Time (min) | Dose given this step (mg) | Cumulative dose (mg) |
|---|---|---|---|---|---|
| 1 | 1 | 2 | 15 | 0.02 | 0.02 |
| 2 | 1 | 5 | 15 | 0.05 | 0.07 |
| 3 | 1 | 10 | 15 | 0.1 | 0.17 |
| 4 | 1 | 20 | 15 | 0.2 | 0.37 |
| 5 | 2 | 5 | 15 | 0.5 | 0.87 |
| 6 | 2 | 10 | 15 | 1 | 1.87 |
| 7 | 2 | 20 | 15 | 2 | 3.87 |
| 8 | 2 | 40 | 15 | 4 | 7.87 |
| 9 | 3 | 10 | 15 | 10 | 17.87 |
| 10 | 3 | 20 | 15 | 20 | 37.87 |
| 11 | 3 | 40 | 15 | 40 | 77.87 |
| 12 | 3 | 75 | 184.4 | 922.13 | 1000 |
Full dose equals 1000 mg. Total protocol time is 349.4 minutes(roughly 5.8 hours). Every step through step 11 runs 15 minutes; the final step runs much longer (184.4 minutes) to deliver the bulk of the total dose once the patient has tolerated everything leading up to it.
Compare this to the "Immunologic IgE" row above: initial dose in micrograms, duration in hours. Step 1 here delivers 0.02 mg, which is 20 micrograms, and the whole protocol wraps up in under 6 hours. The concentration steps up across three progressively stronger solutions, and within each solution the infusion rate climbs before switching to the next, more concentrated bag. Both the rate and the concentration are escalation levers, and the protocol uses both.
The mechanisms table describes the IgE outcome as rendering mast cells "less responsive to degranulation," language that describes a temporary, actively maintained state rather than a permanent fix. That framing is why these protocols are run fresh, under monitoring, each time the drug is needed, rather than treated as a one-time procedure.
| Parameter | When | Watching for |
|---|---|---|
| Wheal diameter, prick test sites | 15-20 minutes after placement | A change >3 mm vs. the negative control, defining a positive result |
| Bleb/wheal size, intradermal sites | 20 minutes after injection | An increase >2 mm from baseline, or >2 mm vs. the negative control |
| Negative (saline) control reaction | Both the prick and intradermal steps | Any unexpected reactivity, which makes the test uninterpretable and may require repeating or escalating to the ID team |
| Positive (histamine) control reaction | Prick test step | Absence of a reaction, which raises concern for a falsely negative overall result |
| Patient tolerance during oral challenge | Throughout the 30-45 minute monitored observation | Any allergic or clinical reaction to the target drug |
| Patient tolerance during IV desensitization | At every 15-minute step, and continuously through the extended final step | Any breakthrough reaction, which would require stopping or slowing the escalation |