HIV testing · timing
A negative test taken too early tells you nothing at all. Which test you need depends entirely on how many days have passed, and most clinics do not say which test they are selling you.
For the first week, no test on earth can find it. After exposure the virus replicates locally before it reaches the bloodstream. During that period, called the eclipse phase, even the most sensitive molecular test has nothing to detect. The median length is about 11.5 days, with a range of roughly 7 to 21.
This is why testing three days after a worrying night is wasted money. The result will be negative, and it will be negative whether or not you were infected.
What changes with time is which biological marker exists to be found. Viral RNA appears first, at around 10 to 14 days. The p24 antigen follows about five to seven days later. Antibodies come last. Each test looks for a different one of those markers, which is what sets its earliest useful date.
Days after exposure
Detects the virus itself. The only test with anything to find before day 18.
Antibody plus p24 antigen, on blood drawn from a vein. The standard screen.
Same principle by fingerstick, but far less sensitive in the acute phase.
Antibodies only. No antigen, so it lags the 4th generation by weeks.
Lowest antibody concentration of any specimen. Treat 90 days as the floor.
The left edge is the earliest it can detect anything. The right edge is when a negative becomes conclusive. A test taken inside its own window can find an infection, but a negative there does not rule one out. That is the distinction that gets lost when a clinic says "wait a month".
Same generation, different sensitivity
A 4th-generation test run on venous blood in a laboratory detects the p24 antigen with a clinical sensitivity above 99 percent. The same generation delivered as a 20-minute fingerstick is a different proposition: independent evaluations put its sensitivity for acute, pre-seroconversion infection somewhere between 40 and 77 percent.
Both are honestly described as "4th generation". If you are testing at day 20 because you are worried about a specific exposure, that gap is the whole ballgame, and it is the reason a laboratory draw beats convenience at the early end of the window.
Oral fluid self-tests sit further out still. Antibody concentration in oral fluid is lower than in blood, which stretches the window rather than shortening it. For a four-week-old exposure, an oral test is the wrong tool.
Prophylaxis changes the biology
A 28-day course of antiretrovirals suppresses viral replication, which blunts both the p24 antigen and the antibody response. If PEP fails, those markers arrive late.
A 30-day test is not enough. Follow-up runs to 3 months, sometimes 6.Acquiring HIV while taking PrEP produces partially suppressed replication. The viral load never surges, so the immune system may never mount a detectable antibody response. Standard 4th-generation tests can stay negative indefinitely.
Oral PrEP in the last 3 months, or injectable cabotegravir in the last 12, means an RNA test is required, not optional.The second case has a name, LEVI syndrome, for Long-acting Early Viral Inhibition. It matters here because PrEP is widely and cheaply available in Bangkok, and a clinic selling a ฿450 antibody test has no way of knowing it is the wrong test unless you say what you are taking. Tell them.
17 clinics with a published HIV test price
The other 15 list "HIV Test", "HIV Testing" or "HIV Screening" at anything from ฿200 to ฿1,700, with no indication of generation or method. Since the generation is what determines whether a result at day 20 means anything, that is the one detail you actually need in order to choose, and it is the detail most often missing.
| Clinic | Test named | Price | City |
|---|---|---|---|
| SWING Silom Polyclinic | 4th generation | ฿450 | Bangkok |
| PSK Clinic | 4th generation | ฿1,000 | Bangkok |
| PrEP by CWC | PCR / NAT | ฿3,500 | Chiang Mai |
| Medpro Laboratory Clinic | HIV-1 PCR | ฿3,200 | Bangkok |
The test you need earliest is the hardest one to price-shop. Only one clinic in our dataset publishes a price for a standalone diagnostic HIV NAT, and it is in Chiang Mai. Medpro lists an HIV-1 PCR at ฿3,200 on its own site. If you are inside the first three weeks and want a molecular test in Bangkok, you will be phoning to ask.
If the exposure was within the last week, wait. Testing now answers nothing. If it has been 10 days to 3 weeks and you need an answer, ask specifically for an RNA or PCR test, and expect to pay several times the price of an antibody test. From about day 18, a laboratory 4th-generation test is the standard screen, and by day 45 a negative from one is conclusive for most people.
If you took PEP, or you are on PrEP, say so before anyone picks a test. In both cases the usual timeline does not hold, and in the PrEP case the usual test can be wrong.
And if the window is still open, the PEP question matters more than the testing question: it is only useful within 72 hours of exposure, which is a far tighter deadline than any of the numbers above.
Where these numbers come from
The window periods, the Fiebig staging sequence, the rapid-test sensitivity figures and the PEP and PrEP guidance are drawn from our own research compilation of the published clinical literature, principally the CDC and APHL laboratory testing algorithm and the Fiebig and EDDI frameworks for early infection staging. Prices and test naming come from the clinics' own published listings, read on 25 September 2026 and recorded with their source URLs.
This is a research document, not clinical advice. Window periods are population ranges, not promises about an individual. The classification of which test suits which timepoint has not yet been reviewed by a clinician. Testing decisions after a real exposure belong with a doctor who knows your history, and if you are inside 72 hours, that conversation is urgent.
Clinical windows: see also panel composition
Prices: clinic listings observed 25 September 2026