Cultures grow bacteria from a sample (such as urine, a wound swab, or blood) and test them against different antibiotics. This tells the clinician whether bacteria are really present and which medicines they are sensitive or resistant to.
This targeted approach means you are more likely to get an antibiotic that works first time, avoid drugs that would not help, and reduce the risk of fuelling resistance with broad, unnecessary prescribing.
Sometimes a doctor will start a best-guess antibiotic while waiting for results, then adjust once the culture comes back. If your symptoms worsen while waiting, or you feel very unwell, contact your clinician or seek urgent care.