Throat Swab Collection
A pharyngeal swab starts every strep workup — technique drives both test paths.
- 2: Swab sites (tonsils + pharynx)
- ~15%: GAS carriage rate (asymptomatic kids)
- 4: Centor criteria (clinical score points)
- 5–15: Peak age group (years old)
Why swab technique matters
Poor swabbing under-samples bacteria and lowers both test sensitivities.
Vigorous tonsillar contact matters more than swab brand.
Who gets swabbed
Centor/McIsaac score guides testing decisions in sore throat patients.
One swab, two paths
A single sample can feed both rapid antigen and culture workflows.
Rapid Antigen Test vs Culture Plate
Same swab, two very different diagnostic philosophies — speed versus accuracy.
- Lateral flow: Rapid test format (antigen capture)
- Sheep blood agar: Culture medium (selective growth)
- ~10 min: Rapid turnaround (point of care)
- 24–48h: Culture turnaround (incubated plate)
Rapid antigen detection
Detects group A carbohydrate antigen directly from the swab extract.
Throat culture
Streaked plate incubates, colonies identified by hemolysis pattern.
Resource tradeoff
Rapid test costs a visit; culture costs a day of waiting.
Many clinics run both in parallel for negative rapid results.
Rapid Test Result in ~10 Minutes
Speed enables same-visit treatment, but sensitivity gaps risk missed cases.
- 70–90%: Typical sensitivity (varies by brand)
- ~95–98%: Typical specificity (few false positives)
- 10–30%: False-negative rate (depends on bacterial load)
- 5–10 min: Result window (lateral-flow read time)
Why negatives happen
Low antigen load or sampling error can hide a true infection.
High specificity, real value
A positive rapid result is trusted enough to treat immediately.
Positive predictive value stays high even at low prevalence.
The backup-culture rule
Negative rapid tests in kids often get confirmed by culture.
Culture Incubates 24–48 Hours
Slower, but culture remains the reference standard for strep confirmation.
- ~95–97%: Culture sensitivity (gold standard)
- ~99%: Culture specificity (colony confirmation)
- 35–37°C: Incubation temp (aerobic/CO2)
- Beta-hemolysis: Colony read (zone around growth)
Why culture wins on accuracy
Live growth confirms viable organisms, not just surface antigen.
The cost of waiting
Two days of delay means empiric treatment decisions happen first.
Confirmatory role
Culture catches rapid-test false negatives before they cause harm.
Missed strep risks rheumatic fever in vulnerable populations.
Clinical Decision Point
Test results funnel into one of three treatment pathways for the patient.
- Rapid +: Treat now (start antibiotics)
- Rapid −, high risk: Wait for culture (confirm first)
- Both negative: No treatment (likely viral cause)
- Prevent: Antibiotic goal (rheumatic fever)
Treat on positive rapid
High specificity justifies immediate empiric antibiotic therapy.
Confirm negatives in kids
Pediatric guidelines favor backup culture after negative rapid tests.
Balancing speed and accuracy
Combining both tests optimizes early treatment and diagnostic safety.
Parallel testing captures speed and sensitivity together.