SPT results should be appropriately interpreted based on clinical symptoms, medical history, and, where necessary, other test results (specific IgE antibody measurements) in order to assess possible allergy to a specific allergen. The probability of a given sensitization to be clinically relevant depends on the type of allergen and country where the patient lives . The clinical relevance of any detected sensitization should be determined by an allergologist after taking a complete history and performing a physical examination. When SPT results and the history are inconclusive, provocation tests may help to determine the clinical relevance of the SPT sensitization, e.g., before initiation of a specific immunotherapy.
SPT is highly specific and sensitive, 70-95% and 80-97%, respectively, to diagnose inhalant allergies . The positive predictive value to diagnose allergic rhinitis based only on the clinical history is 77% for persistent allergy and 82-85% for intermittent seasonal allergy . This increases to 97-99% if SPT is utilized .
The negative predictive value of a negative SPT and in vitro IgE antibody test for cat allergen are identical at 72-75% for cat allergy . A negative SPT for Dermatophagoides pteronyssinus has a negative predictive value in older adults of 90%-95%. However, the positive predictive value ranges from 29% to 43% in older subjects and 77% to 100% for younger subjects .
Sensitivity and specificity are lower for food allergens, ranging from 30-90% and 20-60%, depending on the type of allergen and methods utilized, i.e. pricking with extracts vs. prick-to-prick techniques described earlier . Double-blind placebo-controlled challenge studies in children demonstrate that SPT possesses a positive predictive value of 76% and 89% for clinical reactions to cow’s milk and hen’s egg, respectively .
The objective value of SPT for drug allergy depends on the tested drug. In most cases, a positive SPT makes drug allergy very probable; whereas a negative result does not necessarily indicate that the patient will not react on challenge to the drug . However, for penicillin, the negative predictive value is high. In 98.5% of patients with a negative SPT, no type I allergy was observed upon challenge while the remaining 1.5% of patients had mild and self-limiting reactions, e.g., urticaria . In many cases, intradermal testing is appropriate after negative SPT. Some drugs, e.g., muscle relaxants or opioids may cause SPT false-positive results. When evaluating patients for IgE-mediated drug allergy to antibiotics other than penicillin, SPT should be performed with the unadulterated pharmaceutical agent. Late readings (> 24h) of SPTs and especially intradermal skin tests are very valuable in the clarification of adverse drug reactions.
For suspected insect venom allergy, intradermal tests are the primary mode for detecting sensitization. SPT is performed prior to intradermal testing.
Sensitizations to aeroallergens, as measured by SPT, may precede symptomatic allergy. Prospective studies show that 30-60% of such subjects become allergic depending on the type of allergen tested and the time to follow-up [81, 82]. Furthermore, sensitization can exist to an allergen that is no longer clinically relevant.