The pinprick test, formally known as the Skin Prick Test (SPT), is a standard diagnostic procedure used to determine if a person has an immediate allergic hypersensitivity to specific substances, called allergens. The test works by introducing a small amount of a suspected allergen into the skin to see if the immune system reacts. A reaction indicates the presence of pre-formed antibodies, specifically immunoglobulin E (IgE). The process is quick, typically performed in an allergist’s office, and provides reliable results within minutes for identifying triggers of conditions like hay fever, asthma, and food allergies.
Preparing for the Test
A necessary step before undergoing the pinprick test involves temporarily stopping certain medications that could interfere with the accuracy of the results. The body’s allergic reaction is mediated by histamine, and medications designed to block histamine can suppress a true positive reaction. This suppression can lead to a false-negative result, meaning a person might be allergic, but the test fails to show it.
Common over-the-counter and prescription antihistamines must generally be stopped for about five to seven days before the appointment. Certain older tricyclic antidepressants and specific heartburn medications, known as H2 blockers, also possess antihistamine properties and may need to be discontinued for up to two weeks. Patients should always consult with their physician before stopping any medication, especially prescription ones.
The physical condition of the skin in the testing area is another consideration for preparation. The test is typically performed on the inner forearm or the back, and this skin needs to be free of rashes, severe eczema, or psoriasis. People with a skin condition called dermatographism, where simple pressure causes the skin to swell, may not be suitable candidates for the test. In such cases, the mechanical irritation of the prick itself could cause a false positive reaction, making the results unreliable.
The Step-by-Step Testing Process
The clinician prepares the skin, usually on the forearm or upper back, by cleaning it with alcohol. The test site is then marked with a pen, creating a grid of designated spots spaced at least two centimeters apart to prevent reactions from merging. These markings correspond to the specific allergens being tested.
A tiny drop of standardized liquid allergen extract is placed next to each mark on the skin. Two control solutions are also applied to ensure the test’s validity: a negative control, which is typically a saline or glycerin solution, and a positive control, which is a histamine solution. The negative control should produce no reaction, while the histamine control should always cause a reaction to confirm the patient’s skin is capable of responding.
The clinician then uses a sterile, single-use lancet to gently prick the skin through each liquid drop. This action only breaches the outermost layer of the skin (the epidermis) and does not typically draw blood. The lancet allows the microscopic amount of allergen to penetrate the skin’s superficial layer, where immune cells called mast cells reside.
After the pricking is complete, the patient waits for a standard period, usually between 15 and 20 minutes, for the reaction to develop. During this waiting time, the mast cells, if sensitized to a particular allergen, will release chemical mediators like histamine. This release triggers the localized physical reaction that the clinician will assess.
Interpreting Positive and Negative Reactions
Once the waiting period concludes, the clinician carefully examines and measures the reaction at each test site. A positive result is visually characterized by the development of a wheal and flare response. The wheal is a pale, raised, firm bump that resembles a mosquito bite, while the flare is the surrounding area of redness, or erythema.
The formation of the wheal is a direct result of histamine release, which causes localized swelling and an increase in blood flow to the area. The size of the wheal is the primary measurement used to determine the test result. A wheal that is three millimeters or larger than the negative control site is generally considered a positive reaction.
The clinician measures the diameter of the wheal in millimeters. The negative control site must show no wheal formation, and the positive control site must show a clear, measurable wheal to confirm the skin’s reactivity was normal during the test. If the negative control reacts, or the positive control does not, the entire test may be considered unreliable.
A positive reaction only indicates the presence of IgE antibodies and a sensitivity to the substance, not necessarily the severity of a person’s real-world symptoms. A patient might have a large wheal to a common allergen but experience only mild symptoms, while another person with a smaller reaction might have more severe clinical problems. A negative result, where no wheal forms, indicates a lack of immediate allergic sensitization to the tested substance.
Safety Considerations and Next Steps
The pinprick test is considered a safe procedure, but patients may experience some minor, localized side effects at the test sites, including itching, redness, and swelling. These reactions are temporary and generally resolve within a few hours. If necessary, a mild topical steroid cream or an oral antihistamine can be administered to soothe the skin.
A rare but possible risk is the occurrence of a systemic reaction, known as anaphylaxis, because the test introduces an allergen into the body. To manage this minimal risk, testing is always performed in a medical setting with staff trained to recognize and treat a severe reaction. Emergency equipment, including injectable epinephrine, is readily available, and small, standardized allergen doses help to keep the risk very low.
Once the results are interpreted in conjunction with the patient’s medical history and symptoms, the clinician will outline the next steps. A confirmed diagnosis allows for the development of tailored management strategies, which might include avoidance of the identified allergens. For environmental allergies, the results may also indicate that the patient is a candidate for allergen immunotherapy, commonly known as allergy shots. Immunotherapy involves a long-term course of injections to desensitize the immune system, thereby modifying the underlying allergic response.