After exposure to SARS‑CoV‑2 and onset of COVID‑19 symptoms, how long does it take for a PCR or rapid antigen test to become positive?

COVID-19 Test Positivity Timeline After Exposure and Symptom Onset

Tests typically become positive on the day symptoms begin, but the optimal testing window is 3 days after symptom onset when false-negative rates are minimized to approximately 20%. 1

Timeline From Exposure to Positive Test

Pre-Symptomatic Period (Days 1-4 After Exposure)

• During the 4 days of asymptomatic infection following exposure, the probability of a false-negative PCR result is extremely high, decreasing from 100% on Day 1 to 68% on Day 4. 1
• Do not rely on testing during this window — viral loads are insufficient for reliable detection even with PCR. 1

Day of Symptom Onset (Day 0)

• On the day symptoms first appear, the median false-negative rate for PCR testing is 38%. 1
• Tests can turn positive immediately at symptom onset, but sensitivity is suboptimal. 2
• Antigen tests achieve approximately 89% sensitivity when performed within the first 5 days of symptom onset, with Day 1 being within this optimal window. 2

Optimal Testing Window (Days 3-5 After Symptom Onset)

• The false-negative rate is minimized at Day 3 after symptom onset (approximately Day 8 from exposure), reaching only 20%. 1
• This represents the single best time point for testing if the goal is to minimize false-negative results. 1
• The Infectious Diseases Society of America recommends testing within 5 days of symptom onset to optimize sensitivity. 1
• Antigen test sensitivity peaks at 89% (95% CI: 83-93%) when performed within the first 5 days. 2, 3

After Day 5 From Symptom Onset

• False-negative rates begin increasing again after Day 8 from symptom onset (Day 3 after symptoms), rising from 21% on Day 9 to 66% by Day 21. 1
• Antigen test sensitivity drops substantially to only 54% after 5 days from symptom onset. 4

Practical Testing Algorithm

For Symptomatic Individuals

• Test immediately when symptoms begin to enable early treatment decisions, recognizing that a negative result may be falsely negative. 2
• If the first test is negative and symptoms persist, repeat testing within the 5-day window (ideally on Day 3 after symptom onset). 1, 2
• Consider molecular NAAT (PCR) over antigen testing for confirmation if initial antigen test is negative. 1, 2

For Asymptomatic Exposed Individuals

• Testing during the first 4 days after exposure has limited utility due to extremely high false-negative rates. 1
• If testing is required, wait until Day 5-8 after exposure (corresponding to Days 0-3 after potential symptom onset). 5
• The probability of testing positive peaks between days 5 and 8 after close contact. 5

Critical Caveats

Common Pitfall: Waiting Too Long to Test

• Do not delay testing beyond Day 5 of symptoms — viral loads decline and test sensitivity drops significantly. 1, 4
• Many COVID-19 therapies must be initiated within 5 days of symptom onset, making early diagnosis essential. 2

Test Type Matters

• PCR/NAAT testing remains more sensitive than antigen testing throughout the illness course. 1, 4
• Antigen tests have lower sensitivity in the first week, with approximately 30% sensitivity for antibody testing at days 1-7. 3
• A positive antigen test does not require confirmation due to high specificity (>98%). 3

Special Populations

• Severely immunocompromised patients may have prolonged viral shedding up to 20 days and atypical testing patterns. 4, 3
• Asymptomatic individuals may clear virus more quickly than symptomatic patients. 3

Duration of Test Positivity

• Most people test positive on antigen tests for 5-10 days after symptom onset, with 80.2% remaining positive at Day 5. 3, 6
• PCR can remain positive for 3+ weeks due to detection of non-viable viral RNA fragments. 3