The accuracy of an HIV combo test after 28 days depends on several factors:
Type of test:
- 4th generation tests: This is the most common type of HIV combo test used today. They are extremely accurate, with sensitivity (detecting true positives) exceeding 99% and specificity (detecting true negatives) also exceeding 99% at 28 days after potential exposure.
- Rapid tests: While convenient, these tests generally have slightly lower accuracy compared to laboratory 4th generation tests. Their sensitivity at 28 days may range from 95% to 100% depending on the specific test.
Individual factors:
- Time since potential exposure: Even the most accurate tests take time to detect HIV after infection. While 28 days is considered reliable for most 4th generation tests, 90 days after potential exposure remains the standard for conclusive results.
- Immune system health: Individuals with weakened immune systems may take longer to develop detectable levels of HIV antibodies or antigens, potentially affecting test accuracy at 28 days.
Therefore, while a negative result at 28 days with a 4th generation laboratory test is highly suggestive of being HIV-negative, it's not considered conclusive until confirmed by a repeat test at 3 months (90 days) after the potential exposure.
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