Lincoln Diagnostics Inc

For many years, histamine at a concentration of 1 mg/mL has been used as a positive control when conducting percutaneous skin testing. Currently, histamine is also being used at a concentration of 6 mg/mL. Increasing the concentration by 6 times raises the following issues. First, if the reaction from histamine is considered a 3 response, then the sensitivity from extract results is lowered. Dykewicz et al 1 reported a mean wheal size of 9.23 mm from 6 mg/mL of histamine when administered by Multi-Test II. Berkowitz et al 2 reported a mean wheal size of 6.7 mm from 1 mg/mL of histamine when applied by Multi-Test. According to these comparative findings, the mean wheal size from 6 mg/mL of histamine was 38% larger than the mean wheal size produced from 1 mg/mL of histamine. With 6 mg/mL of histamine, a 3 reaction from an extract would be a wheal that measures approximately 9 mm. With 1 mg/mL of histamine, the wheal would have to measure approximately 7 mm to be classified as 3. If histamine at a concentration of 6 mg/mL is to be used, the grading standards for extract reactions would have to be altered. One cannot simply change the concentration of histamine without dealing with the concentration of diagnostic extracts and how wheal sizes from them compare with positive control and patient symptoms. Respected publications 3,4 support the use of histamine at a concentration of 1 mg/mL as a positive control. Second, patients who are taking medications that suppress wheal reactions could show wheal reactions from histamine at a concentration of 6 mg/mL but negative results from histamine at a concentration of 1 mg/mL. An occurrence such as this could lead to many false-negative results from diagnostic extracts. It is hoped that the issue of proper histamine concentration, as a positive percutaneous skin test control, will be addressed by both the College and the Academy.