Where contamination enters
- Skin and mucosal flora carried into the sample by the needle or swab
- The collector's hands or a non-sterile glove touching the specimen or container
- Non-sterile containers, reused tubing ports, or open collection systems
- Environmental organisms entering an uncovered or mishandled sample
What delay and temperature do
Transport medium and time-to-laboratory are not administrative details; they decide which organisms survive to be identified. Fastidious organisms such as Neisseria species and anaerobes die quickly when a specimen dries out, cools, or is exposed to oxygen. Meanwhile, organisms that tolerate those conditions, including many environmental gram-negative bacilli, continue to multiply. A specimen that sat at room temperature for hours can therefore yield a population dominated by contaminants while the true pathogen is no longer recoverable.
Common reasons a specimen is rejected
- Wrong specimen type for the suspected infection, such as a surface swab submitted for a deep infection
- Unlabeled or mislabeled container, or a label that does not match the request
- Leaking, broken, or unsealed container that compromises sterility
- Dry swab submitted for an organism that requires transport medium
- Excessive delay or improper storage that makes recovery unreliable
- Material that cannot be cultured meaningfully, such as saliva submitted as sputum
Reading a problem specimen
A sputum sample that is mostly saliva and squamous epithelial cells on direct examination indicates that the lower respiratory tract was not sampled; the culture will grow oral flora and cannot identify a pulmonary pathogen. A blood culture bottle that arrives warm after sitting in a vehicle for six hours may grow an environmental organism that entered during handling, while a fastidious pathogen may have died. In both cases the laboratory's correct action is to reject or qualify the specimen rather than report a result that would mislead therapy.