Strep A Antigen (Rapid Strep Test)
Definition
Detects the group A streptococcus bacterium that causes throat infection (streptococcal pharyngitis). There are two methods:
| Method | What it looks for | Turnaround time | Accuracy |
|---|---|---|---|
| Rapid strep test | Strep A antigens | 10 to 20 minutes | Lower |
| Throat culture | The Strep A bacterium | 24 to 48 hours | Higher |
Purpose
In the presence of sudden, severe sore throat, difficulty swallowing, fever of 38.3 °C or higher, swollen lymph nodes, red tonsils with white patches, or the rough red rash characteristic of scarlet fever.
Results
| Result | What is done |
|---|---|
| Rapid test positive | Strep throat infection confirmed, no further testing needed |
| Rapid test negative but clinical suspicion remains | A throat culture is ordered for confirmation |
| Culture positive | Infection confirmed, antibiotics needed |
| Culture negative | A viral infection is considered |
Risks and Limitations
The rapid test has a sensitivity limit:
A throat culture sometimes finds infections that the rapid strep test misses, which is particularly important in children and adolescents at risk of rheumatic fever.
In other words, a negative rapid test alone is not sufficient in children.