The Mechanism Behind Antihistamine Interference
Allergy skin prick testing works by introducing a tiny amount of a potential allergen into the skin's surface. In someone with a true allergy, the immune system reacts by releasing histamine at the site of the test. This release of histamine causes a localized allergic reaction, typically appearing as a raised, red bump called a "wheal," surrounded by a larger red area known as a "flare." The size of the wheal and flare helps the allergist determine the severity of the allergic reaction to that specific substance.
Antihistamines are medications designed to block the effects of histamine. When these drugs are active in your system, they prevent the visible and measurable histamine-induced wheal and flare reaction from occurring, even if you are truly allergic to the substance being tested. This interference means the allergist may see little to no reaction, leading to a false-negative result and a missed diagnosis. For this reason, it is crucial to stop taking antihistamines before the test, following your doctor's specific instructions.
The Consequences of a False Negative Result
Ignoring the pre-test instructions about antihistamine withdrawal can have serious consequences. The most significant is the risk of a false-negative diagnosis. If your allergy test shows no reaction because of antihistamine use, you might be told you do not have an allergy when you actually do. This can prevent you from receiving the proper treatment, such as prescription medication or allergy immunotherapy (allergy shots or drops).
A delayed or incorrect diagnosis can lead to continued exposure to the allergen, which may exacerbate your symptoms and affect your quality of life. Furthermore, if the test is unreliable, you may need to schedule and undergo a second round of testing, causing further delay and inconvenience. For conditions like severe allergic asthma or potential food allergies, a missed diagnosis can have life-threatening implications.
How Long to Stop Antihistamines Before Testing
Withdrawal times vary significantly depending on the type of antihistamine and its half-life, which is the time it takes for half of the drug to be eliminated from the body. Different medications require different periods to be fully cleared from your system. It's always best to consult with your allergist, but general guidelines are commonly provided.
Common Antihistamine Withdrawal Periods
- Second and Third-Generation Oral Antihistamines: These typically require discontinuation for 5 to 7 days before testing. Examples include cetirizine (Zyrtec), loratadine (Claritin), fexofenadine (Allegra), and levocetirizine (Xyzal).
- First-Generation Oral Antihistamines: These generally require a shorter withdrawal period, often around 48 to 72 hours, though some, like hydroxyzine (Atarax, Vistaril), may need up to 10 days. A common first-generation antihistamine is diphenhydramine (Benadryl).
- Antihistamine Nasal Sprays and Eye Drops: These topical forms also affect test results and should be stopped, often 1 to 5 days before testing. Examples include azelastine (Astelin, Astepro) and olopatadine (Patanase, Pataday).
- Certain H2 Blockers: Medications for heartburn like ranitidine (Zantac), famotidine (Pepcid), and cimetidine (Tagamet) can also interfere with skin tests and require a withdrawal period, usually 1 to 2 days.
- Other Medications: Be aware that many cold, sinus, and sleep aids contain antihistamines, and some antidepressants and anti-nausea medications also have antihistamine effects. It is crucial to read labels carefully and inform your allergist of all medications you are taking.
Comparison of Common Antihistamine Withdrawal Times
| Type of Antihistamine | Examples | Typical Withdrawal Period |
|---|---|---|
| Second/Third-Gen. Oral | Cetirizine (Zyrtec), Loratadine (Claritin), Fexofenadine (Allegra), Levocetirizine (Xyzal) | 5–7 days |
| First-Gen. Oral | Diphenhydramine (Benadryl), Chlorpheniramine (Chlor-Trimeton) | 48–72 hours |
| Hydroxyzine | Atarax, Vistaril | Up to 10 days |
| Nasal Spray (Antihistamine) | Azelastine (Astelin, Astepro), Olopatadine (Patanase) | 1–5 days |
| H2 Blockers (Certain Antacids) | Famotidine (Pepcid), Cimetidine (Tagamet) | 1–2 days |
Alternatives to Skin Prick Testing
If you find it impossible to stop your antihistamine medication due to severe symptoms, or if there is a concern about other medications (like beta-blockers, which affect the treatment of a severe allergic reaction), your doctor may recommend a blood test for allergies instead.
A serum-specific IgE blood test measures the amount of specific IgE antibodies in your blood that are created in response to a particular allergen. Since this test does not rely on a localized skin reaction, antihistamines and other medications do not interfere with the results. While generally more expensive and less immediate than skin testing, a blood test provides a reliable alternative when stopping antihistamines is not feasible.
What if You Forgot to Stop Your Antihistamines?
If you realize you took an antihistamine too close to your scheduled allergy test, you should inform your allergist's office immediately. They will advise you on the next steps, which will likely involve rescheduling your appointment. It is never wise to proceed with testing knowing the results will be compromised, as this can lead to an incorrect diagnosis. The inconvenience of rescheduling is far outweighed by the importance of getting an accurate result the first time.
After the Allergy Test
Following the completion of your allergy test and the reading of the results, you can typically resume your antihistamine regimen as normal. The exact timeline may depend on your specific treatment plan, especially if you are starting allergy immunotherapy. Your allergist will provide you with clear instructions regarding medication use after the test.
List of Medications to Watch Out For
- Common oral antihistamines: Cetirizine (Zyrtec), Loratadine (Claritin), Fexofenadine (Allegra), Levocetirizine (Xyzal), Diphenhydramine (Benadryl), Hydroxyzine (Atarax, Vistaril), Chlorpheniramine (Chlor-Trimeton).
- Certain cold, cough, and sinus medications: Many over-the-counter options combine a decongestant with an antihistamine, such as Tylenol PM, Nyquil, or Advil PM.
- Antihistamine eye drops: Olopatadine (Patanol, Pataday), Azelastine (Optivar).
- Antihistamine nasal sprays: Azelastine (Astelin, Astepro), Olopatadine (Patanase).
- Some antacids: Cimetidine (Tagamet), Famotidine (Pepcid), Ranitidine (Zantac).
- Some antidepressants: Tricyclic antidepressants like amitriptyline (Elavil), doxepin (Sinequan), and nortriptyline (Pamelor).
- Anti-nausea medications: Promethazine (Phenergan), Meclizine (Antivert, Bonine).
Conclusion
Taking antihistamines before a skin prick allergy test can significantly alter the results, potentially leading to a false negative and an incorrect diagnosis. It is essential to follow your allergist's instructions for withdrawing from these medications, which typically involves stopping them for several days, depending on the specific drug. For those who cannot stop their medication due to severe symptoms, a blood test for allergies offers a reliable alternative. Always communicate openly with your healthcare provider about all medications you are taking to ensure the most accurate and effective outcome for your allergy testing.
