Culture means seeing if a germ grows from a specimen. Sensitivities (or “Susceptibilities”) means that once a germ is found, tests are done to compare antibiotics to see which work on that specific germ. The C&S is usually done for bacteria; cultures by themselves can be done to grow any kind of germ (see Differences Among Germs). Note that while a culture takes at least a day for results, we can get a more rapid idea by ordering a Gram Stain of the specimen, when the lab tech rapidly stains a slide from it & examines it under the microscope. But that only tells us a little.
Just because a bacteria grows in culture doesn’t mean it’s the cause of infection. For example, sputum (phlegm) coughed up might pick up normal mouth germs. Urine cultures might find germs from the skin or vagina. And even though we clean the skin with antiseptic before drawing blood cultures, sometimes they’re contaminated by normal skin germs. The elderly, and anyone with a urinary catheter, may well have bacteria living harmlessly in the bladder, so just because they grow in culture doesn’t mean they’re causing illness. The term for this is “colonization“.
Sensitivity testing shows which drugs the bacteria is sensitive to (they’ll work), and to which it’s resistant (they won’t work). This is usually accurate, but sometimes so much of a “resistant” drug gets excreted into the urine that it may work for bladder infections (but not kidney infections, since they require drug to penetrate the entire kidney by way of the bloodstream).
In summary, the C&S usually tells us what germ is causing an infection, and what drugs will work best against it. The problem is that it can take 2 or more days for final results (sometimes more if the lab doesn’t staff enough techs 24/7). For some germs that grow slowly, like TB, it can take weeks or even a couple of months for results. So clinicians have to do a lot of best-guessing in the meantime.
We don’t always do a C&S even though there’s an infection. For example:
- Bladder infection (UTI) — it’s quicker & cheaper to just treat for a few days (we do obtain urine culture for kidney infections, which are more dangerous).
- Pneumonia — Studies show that cultures are not reliable, with lots of false-positives and false-negatives. There are treatment strategies that don’t rely on culture.
- Diarrhea — Stool cultures can find bacteria like Shigella, Salmonella, & Campylobacter, but not viruses nor parasites. Bacteria aren’t usually the cause, but if we think they may be, we have to wait for full results with antibiotic sensitivities.
- Sinusitis — Very hard to easily & safely get a specimen from the sinus itself. A specimen from the nose is useless; too many other harmless bacteria live there.
- Cellulitis — A skin infection that’s impossible to culture.
- Tooth Infection — No way to find the true germ at the root, & several are usually infecting the tooth together (all successfully treated by same antibiotics plus extraction or root canal).