DFA can be used to detect B. pertussis directly on smears prepared from nasopharyngeal specimens. It provides more rapid results than culture.
However, DFA lacks both sensitivity and specificity. The sensitivity of DFA methods is dependent on when the specimen is collected during the course of the disease. These methods are most useful in the first 2-3 weeks of illness. False-negatives may be caused by inadequate specimens containing minimal cellular material (leukocytes and brush border epithelial cells).
DFA is best used in combination with culture and PCR to confirm a diagnosis of B pertussis.