MRSA test: how it works and why it's done
MRSA testing answers two questions: are you carrying the bacteria on your skin or in your nose, and — if you are ill — is MRSA the cause? The nasal swab, PCR, and bacterial culture each address a different part of the picture.
The short answer
An MRSA test is usually a quick swab of the inside of the nose, sometimes together with the throat, groin, or an open wound. The swab is analysed either by rapid PCR (1–2 hours, detects the resistance gene) or by traditional culture (24–72 hours, grows the bacteria for a full antibiotic susceptibility panel). For suspected invasive disease, blood, urine, sputum, or joint fluid is cultured instead.
For why the nostrils are swabbed and what nasal carriage means, see MRSA in the nose →
Why MRSA testing is done
Testing serves three distinct purposes and it helps to keep them separate. Diagnostic testing confirms whether a symptomatic infection — a wound, abscess, pneumonia, bacteraemia — is caused by MRSA and guides which antibiotic to use. Screening is done on people who are well but at higher risk of carrying MRSA, so precautions can be taken before an infection develops. Common screening triggers are admission to hospital, planned surgery, dialysis, ICU care, and transfer from another facility. Clearance testing is done after a course of decolonisation to see whether the bacteria have been eradicated.
How the test is done
The classic MRSA test is a nasal swab. A cotton-tipped swab is rotated inside each nostril for a few seconds. It is painless and takes under a minute. For higher-sensitivity screening, the throat, groin, perineum, and any open wound may also be swabbed — around 15–20% of MRSA carriers colonise sites outside the nose alone.
When an active infection is suspected, the sample is taken from wherever the infection sits: pus from an abscess, sputum from suspected MRSA pneumonia, blood from a spiking fever, urine, joint fluid, or bone. Two or more sets of blood cultures are drawn from separate sites when bacteraemia is on the table.
Types of MRSA test
| Test | Sample | Turnaround | Used for |
|---|---|---|---|
| Nasal-swab PCR | Anterior nares (both nostrils) | 1–2 hours | Hospital admission screening; pre-surgical screening |
| Bacterial culture + susceptibility | Wound, abscess, sputum, blood, urine, joint fluid | 24–72 hours | Confirming active infection and guiding antibiotic choice |
| Blood culture | Two or more sets of blood | 24–72 hours | Suspected MRSA bacteraemia, endocarditis, or sepsis |
| Multi-site screening swabs | Nose, throat, groin, perineum, wounds | 24–48 hours (culture) or 1–2 hours (PCR) | High-risk carriage screening and clearance testing |
PCR vs culture
PCR tests amplify DNA and look specifically for the mecA or mecC gene — the genetic marker that makes S. aureus methicillin-resistant. Results come back in 1 to 2 hours, which is why PCR dominates admission screening. The drawback is that PCR only answers one question: MRSA or not. It does not produce a live organism or a full susceptibility panel.
Culture grows the bacteria on selective agar over 24 to 72 hours. The isolate is then tested against a panel of antibiotics, producing a report of which drugs the strain is sensitive and resistant to. Culture is slower but essential for guiding treatment of an established infection, especially when second-line drugs are being considered.
What the results mean
A positive nasal or skin swab means colonisation — you are carrying MRSA on your body without symptoms. Carriers can transmit the bacteria and are at slightly higher risk of infection if the skin barrier is broken, but colonisation is not itself an illness. Roughly 2% of the general population is colonised at any given time.
A positive culture from a normally sterile site — blood, joint fluid, cerebrospinal fluid — is an active infection and needs prompt treatment. A positive wound or sputum culture is interpreted in the context of clinical signs; MRSA can colonise wounds and airways without being the cause of illness. More information on our Treatment page →
Accuracy and limitations
Nasal-swab PCR is roughly 90–95% sensitive and over 95% specific for MRSA colonisation. Sensitivity climbs when multiple sites are swabbed. False negatives happen when the swab is taken poorly, when the carrier's dominant site is outside the nose, or when recent antibiotic or antiseptic use has temporarily suppressed the organism. False positives are rare but can occur when the mecA gene sits in a related coagulase-negative staphylococcus rather than in S. aureus itself.
Testing after decolonisation
After a course of decolonisation — typically five days of intranasal mupirocin plus chlorhexidine body washes — clearance testing is done at least 48 hours after the last dose. Multiple negative swabs over successive weeks are usually needed before a carrier is considered cleared, because MRSA can rebound. Read more about decolonisation and prevention →
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