What is the recommended concentration for intradermal (ID) allergy skin testing?

Recommended Concentration for Intradermal Allergy Skin Testing

For intradermal (ID) allergy skin testing, use a 1:1000 (vol/vol) or 1:10,000 (vol/vol) dilution of the maintenance concentrate, with 0.02-0.05 mL injected into the dermis. 1

Standard Testing Protocol

Volume and Technique

• Inject 0.01-0.02 mL of allergen extract into the dermis for intradermal testing 2, 3
• Some protocols use up to 0.05 mL for standardized testing procedures 1
• The injection creates a visible wheal through histamine release from mast cells, with subsequent erythema from vasodilation 2, 3

Concentration Selection Based on Extract Type

For Standardized Extracts:

• Use concentrations in the range of 10-100 BU/mL for intradermal testing 4
• Optimal concentration is approximately 30 BU/mL for standardized grass and dust mite extracts 4
• This provides a cutoff histamine ratio of 0.87 with 87.1% predictive value 4

For Nonstandardized Extracts:

• Start with 1:1000 (vol/vol) dilution of the maintenance concentrate for most patients 1
• Use 1:10,000 (vol/vol) dilution for highly sensitive patients based on history or initial prick test reactivity 1
• Nonstandardized extracts typically come at 1:10 to 1:50 wt/vol or 20,000-100,000 PNU concentrations 1

Sequential Testing Approach

When to Perform Intradermal Testing

• Perform ID testing after negative skin prick testing (SPT) when clinical suspicion remains high 5
• Approximately 20.8% of negative SPT results will show positive ID tests, particularly for indoor allergens like dog (33.3%), cat (34.3%), and dust mites (39.4-39.6%) 5
• ID testing is more sensitive but less specific than prick testing 2, 3

Dilution Series for Drug Allergy Testing

For suspected drug hypersensitivity (e.g., corticosteroids):

• Use 10% stock concentration for skin prick tests 6
• Progress to intradermal testing with serial dilutions: 1:1000,1:100, and 1:10 6
• A wheal ≥3 mm greater than negative control is considered positive 6

Critical Safety Considerations

Maximum Expected Wheal Size

• The maximum safe wheal size for 95% of the population is 26.6 mm for intradermal testing 4
• Larger reactions may indicate excessive concentration or high sensitivity 4

Adjusting for Patient Sensitivity

• Highly sensitive patients (based on history of severe reactions or strong prick test reactivity) require more dilute starting concentrations 1
• The starting dilution should balance diagnostic yield against systemic reaction risk 1

Extract Variability Warning

• Nonstandardized extracts vary widely in biologic activity regardless of weight/volume or PNU labeling and should not be considered equipotent 1
• Standardized extracts are preferred when available (cat, dust mite, grasses, ragweed, Hymenoptera venoms) because biologic activity is more consistent 1

Common Pitfalls to Avoid

• Do not assume equivalent potency between different manufacturers' nonstandardized extracts at the same weight/volume concentration 1
• Do not skip prick testing and proceed directly to intradermal testing, as this increases systemic reaction risk 2, 3
• Do not perform intradermal testing without emergency equipment immediately available, as anaphylaxis can occur 7
• Do not confuse test doses with diagnostic skin testing—test doses (like those for iron dextran at 25 mg IV) serve a different purpose and have limited predictive value for subsequent reactions 7