Culture and Sensitivity
A laboratory test that grows bacteria from a sample and identifies which antibiotics will kill them.
What it means
Culture grows any bacteria present in a sample; sensitivity testing then establishes which antibiotics kill them. Often written MC&S — microscopy, culture and sensitivity.
The report names the organism and lists antibiotics as S (sensitive, will work), I (intermediate) or R (resistant, will not work).
Results take 48 to 72 hours, because bacteria have to be given time to grow. This is why the first antibiotic is a considered guess, based on the likely organism and local resistance patterns, and is changed if the result says otherwise.
In practice
Common samples: urine, wound swab, throat swab, sputum, stool, blood cultures.
"No growth" may mean no infection, a viral cause, or that antibiotics were started before the sample was taken — which is why samples should be collected first where possible.
Why it matters
An important distinction: bacteria growing in a sample from someone with no symptoms usually does not need treating. Asymptomatic bacteriuria in older adults and catheterised patients is the classic example, where treating causes harm and no benefit.
Symptoms not settling on an antibiotic is a reason to check the sensitivity result, not automatically to try a different antibiotic blindly.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026