Staphylococcus aureus Antigen (Serum/Plasma) – Quantitative

What This Test Measures

The test identifies *Staphylococcus aureus* antigen, a bacterial toxin released during infection. It’s not the same as blood cultures, which look for live bacteria. This test is useful for confirming infection when cultures are negative or in cases where rapid results are needed.

Your healthcare provider may order this test if you have symptoms like fever, redness, or pus from a wound, or if you’ve been exposed to *Staphylococcus aureus*. It’s also used in monitoring patients with known infections, such as those with endocarditis or severe skin/soft tissue infections.

No special preparation is needed. You’ll have a small needle inserted into a vein (usually in your arm) to draw blood. The process takes about 5 minutes, and you may feel a brief pinch. If you’re on blood thinners, let your provider know before the test.

A positive result means your blood contains the bacterial antigen, strongly suggesting an active *Staphylococcus aureus* infection. A negative result means the antigen isn’t detected, but infection could still be present (e.g., in tissues not sampled). Follow-up testing may be needed.

A positive result may prompt immediate treatment with antibiotics tailored to *Staphylococcus aureus*, such as vancomycin or linezolid. Your provider will adjust care based on symptoms and lab results.

Common Causes of False Results

False positives are rare but can occur if you’ve recently taken antibiotics or if the test was done too soon after exposure. False negatives may happen if the infection is localized (e.g., in skin) and not in the bloodstream. Your provider will interpret results in context with your symptoms and other tests.