5.4 Allergy Testing & Diagnostic Imaging Modalities
Key Takeaways
- Skin prick (scratch) testing is read at 15 to 20 minutes and is positive when the wheal is at least 3 mm larger than the negative saline control; histamine serves as the positive control.
- Antihistamines must be withheld for approximately 3 to 7 days before skin testing because they suppress the wheal-and-flare response and produce false-negative results.
- Intradermal allergy testing is more sensitive than prick testing but carries higher false-positive and systemic reaction risk, so it is performed after a negative prick test, never before it.
- Patch testing detects type IV delayed cell-mediated contact dermatitis; patches remain in place 48 hours and are read at removal and again at 72 to 96 hours.
- Magnetic resonance imaging uses a strong magnetic field with no ionizing radiation, so the critical screening question concerns ferromagnetic implants, pacemakers, and metallic foreign bodies rather than pregnancy alone.
Allergy Testing
Immediate (type I) allergy is mediated by immunoglobulin E (IgE) bound to mast cells. Introducing a trace of allergen into sensitized skin cross-links IgE, degranulates the mast cell, and releases histamine, producing the wheal-and-flare reaction that all skin testing reads.
Skin prick (scratch) testing
A drop of standardized extract is placed on the volar forearm or upper back and a lancet pricks or scratches through it. Every panel includes two controls:
- Positive control: histamine. If it fails to produce a wheal, the patient is suppressed — usually by antihistamines — and the entire panel is uninterpretable.
- Negative control: saline or glycerin diluent. If it produces a wheal, the patient has dermatographism and the panel will over-read.
Results are read at 15 to 20 minutes. A reaction is positive when the wheal measures at least 3 mm larger than the negative control. The wheal (the raised central welt) is measured, not the surrounding erythematous flare.
Medication holds
Antihistamines are withheld for approximately 3 to 7 days before testing, following the specific interval the allergist orders — first-generation agents such as diphenhydramine clear faster than second-generation agents such as cetirizine, and some tricyclic antidepressants and H2 blockers also suppress the response. Failure to hold antihistamines is the single most common cause of a wasted testing appointment. Verifying the hold when the appointment is confirmed is a medical assistant responsibility. Inhaled and most oral corticosteroids do not need to be stopped for skin prick testing, and asthma medications are generally continued.
Intradermal testing
A small volume (about 0.02 to 0.05 mL) of a more dilute extract is injected intradermally to raise a bleb. It is more sensitive but less specific than prick testing, with more false positives and a higher risk of systemic reaction, so it is performed only after a negative prick test — never as the first step, and generally not for foods.
Patch testing
Patch testing evaluates type IV delayed cell-mediated hypersensitivity — allergic contact dermatitis from nickel, fragrances, preservatives, rubber accelerators, and topical medications. Chambers are applied to the upper back and remain in place for 48 hours. The patient must keep the area dry and avoid sweating, bathing, and sun exposure. The site is read at removal and again at 72 to 96 hours, because delayed reactions frequently appear only at the second reading.
Blood-based specific IgE testing
Specific IgE immunoassay — historically the radioallergosorbent test (RAST) and now generally a fluorescent or enzyme immunoassay — measures allergen-specific IgE in serum. It is the alternative when the patient cannot stop antihistamines, has extensive eczema or dermatographism leaving no clear test surface, has a history of anaphylaxis making skin testing hazardous, or is unable to cooperate with skin testing.
Emergency preparedness
Allergen testing carries a genuine risk of systemic reaction. Epinephrine, a stocked emergency kit, and provider supervision must be immediately available, and the patient is observed for at least 20 to 30 minutes after testing. Any spreading urticaria, wheeze, throat tightness, or hypotension is treated as anaphylaxis.
A patient arrives for skin prick allergy testing. During intake the medical assistant learns she took cetirizine (a second-generation antihistamine) that morning. What is the appropriate action?
Diagnostic Imaging Modalities
The medical assistant does not interpret images, but does schedule studies, obtain prior authorization, deliver preparation instructions, and complete safety screening. Choosing the wrong preparation or missing a screening contraindication cancels the study or endangers the patient.
| Modality | Mechanism | Ionizing radiation | Typical preparation | Key screening question |
|---|---|---|---|---|
| Radiography (x-ray) | Differential absorption of x-ray photons | Yes | Remove metal and jewelry from the field | Pregnancy |
| Computed tomography (CT) | Rotating x-ray beam reconstructed into cross-sections | Yes (higher dose than plain film) | NPO 4 hours if contrast; oral or intravenous contrast may be ordered | Pregnancy; iodinated contrast allergy; kidney function |
| Magnetic resonance imaging (MRI) | Strong magnetic field plus radiofrequency pulses | No | Remove all metal; long scan in an enclosed bore | Ferromagnetic implants, pacemakers, aneurysm clips, metallic foreign body, cochlear implants; claustrophobia |
| Ultrasound (sonography) | High-frequency sound waves and echo timing | No | Full bladder for pelvic/obstetric; NPO 6–8 hours for gallbladder | Generally none; safe in pregnancy |
| Nuclear medicine | Injected or ingested radiopharmaceutical detected by gamma camera | Yes (internal source) | Varies by study; often a waiting period for uptake | Pregnancy and breastfeeding |
MRI is the modality that most often appears on the exam, precisely because its hazard is counterintuitive. It uses no ionizing radiation, so the reflex "is she pregnant" screening question is not the critical one. The critical question is metal: the magnet can displace ferromagnetic implants and aneurysm clips, heat conductive material, and disrupt cardiac devices. Many contemporary implants are labeled MRI-conditional and are safe under specified conditions, which is exactly why the screening form must capture the device make and model rather than a simple yes or no.
Contrast considerations. Iodinated contrast used in CT is screened against contrast allergy and kidney function. Gadolinium used in MRI is screened against severe kidney impairment. Both screens are completed before the appointment, not on arrival, so an abnormal creatinine does not cancel the study on the day.
The full-bladder rule applies to pelvic and obstetric ultrasound: a distended bladder displaces bowel gas and creates an acoustic window. The gallbladder rule is the opposite — the patient fasts so the gallbladder distends with bile rather than contracting. Mixing these two preparations is a frequent distractor.
A patient is scheduled for a brain MRI. Which item on the screening form most urgently requires provider review before the study proceeds?
A patient is scheduled for both a pelvic ultrasound and a right upper quadrant (gallbladder) ultrasound on the same morning. What preparation instruction should be given?