Reasons why a false negative might be suspected:
(1) bacteria seen on Gram stain but not recovered on culture
(2) other cultures positive for the organism
Possible causes of a false negative culture of a non-labile organism:
(1) dried out due to delayed delivery or failure to add transport medium
(2) freezing or excessive heat
(3) presence of an inhibitor (antibiotic, toxin, acid or base)
(4) improper tissue processing
(5) overgrowth by other organisms at site
(6) contamination of the specimen
(7) inappropriate specimen
(8) insufficient material (often a barely moistened swab)
(9) very low numbers of viable microorganisms at the site being cultured
(10) selection of appropriate media
Successful culture requires:
(1) proper site
(2) proper collection
(3) prompt delivery
(4) protected delivery (from drying, contamination, temperature extremes)
(5) proper processing
