Penicilin allergy test عملي

فارما / د.منة / طب عام ثالث

عملي(1) 9/3/2017
Practical Pharmacology-3rd Class- 2016

Penicillin Allergy Skin Testing

- Allergy to penicillin is the most common reported drug allergy, in some studies up to 10% of patients could report some form of penicillin allergy, however approximately 90% of patients who report such allergy are not truly allergic when assessed by proper skin testing.
Although a history of penicillin allergy should always be considered seriously and never be ignored, on the other hand, penicillins are probably the most important group of antibiotics on current daily clinical practice, and unnecessary withholding of these drugs can cause harm to the patient in form of increased morbidity, mortality, and medical cost.
Penicillin skin testing helps to recognize those patients who are truly allergic to penicillin.
Notes:
1- No combination of testing can offer a 100% diagnostic accuracy so it is important to take careful history when evaluating a patient with history of penicillin allergy.
2- Patients with history of recent and severe anaphylaxis to penicillin and negative skin testing should be managed very carefully and if medically appropriate continue with other antibiotics.
3- Patients with severe skin reaction to penicillin should not be exposed to skin testing.

Penicillin Allergy Testing, includes:

Preparing the required supplies.
Performing scratch testing.
Conducting intra dermal testing.
Offering an optional oral challenge.
Required supplies:
1- Proper reagents including:
-Benzylpenicilloyl (the major antigenic determinant in penicillin allergic individuals)
- Dilute Penicillin G 10,000 U/ml.
- Histamine for positive control.
- Sterile diluents negative control.
Note: It is important to make sure that the patient does not take any form of anti histamine during the last 48 hours.
2- Scratch testing device.
3- 26- 28 gauge syringe with label.
(Label each syringe before withdrawing the reagent)
4- Alcohol swabs and skin marking pen.


Note: Although rare, skin testing can induce anaphylaxis, so it is important to have the facility to treat anaphylactic shock, so there should be:
- Stethoscope and sphygmomanometer.
- Adrenaline 1:1000
- Hydrocortisone.
- Antihistamines.

Methods:

- Begin with Scratch testing as a screening test; this is conducted with diluted Penicillin G, Benzylpenicilloyl, Saline (as a negative control), and Histamine ( as positive control).
1- Use the ventral aspect of the patients forearm,
2- Clean the skin with alcohol swabs to prepare the skin surface.
3- Mark the skin to clearly identify where each reagent will be placed.
Note: Avoid cross contamination.
4- Place one drop of the proper reagent on each area, after that use a scratch testing device to scratch the surface of the skin.
5- Apply minimum pressure to break the top layer of the skin.
6- Read the results within 15-20 minutes, but never delay reading as the results can diminish after 20 minutes.
7- Read the histamine result first as a positive reaction to histamine is required to move forward in reading the test.
a- If you do not see a reaction to histamine within 15 minutes the patient has likely been taking antihistamine and the test should be stopped and conducted on another day because results will not be valid.
b- If the patient reacted to histamine by appearance of wheel and some flare the test is valid and you can move forwarded in reading the results.
Conclusion:
- A positive test will result in induration of greater than 3 mm than the negative control.
- If the test is positive the patient is labeled as allergic to penicillin. (SO: No need to go to dermal testing).
- If the result of scratch test is negative you can proceed to intra-dermal testing.


Intra-dermal testing:
This is conducted with Benzylpenicilloyl, Penicillin G and negative diluents control.
1- Clean the skin with alcohol and mark the test area appropriately as with scratch test.
2- Raise a 2-3mm bleb using the proper reagent, then mark the edge of it by circling its parameter.
3- Watch for any change outside that original bleb, the back of the upper arm can be used instead of the forearm if it was more comfortable to the patient.
4- Read the results within 15-20 minutes as with the scratch test.

Conclusion:

Negative test: If there was no increase in the size of the original bleb, and no reaction greater than the control site, the test is reported as
A positive test: is characterized by:
1- Increase in the size of the original bleb with wheel diameter of 3mm or larger than the diluents control.
2- Itching and flare are commonly present.
If equivocal result were observed with wheel only slightly larger than the original injection bleb, then the test can be repeated.
Negative test: If there was no increase in the size of the original bleb, and no reaction greater than the control site.
- After a negative test, an oral challenge with oral dose of penicillin can be started with careful observation of the patient for about one hour.
Note: Oral challenge is safer than injectable dose challenge.