HIV Testing: Detection Limits and Measurement Units Explained

HIV Testing

Hello, I would like to ask about my HIV PCT blood test results, which show no infection.
However, the report states "HIV detection limit: 33 IU/mL, Roche Cobas 6800 System.
(1 copy = 1.67 IU)." I want to know that the current medical detection limit is 20 copies/mL (less than 20 viral particles).

First question: The report does not specify this.
I have found various versions stating that 1 copy = 0.6 IU, 1 copy = 5 IU, and 1 copy = 5.8 IU.
Which one is correct?
Second question: The report mentions "copy." Is it the same as "copies"?
Third question: This is a hospital test.
Is the minimum detection limit of 33 IU the lowest possible?

Ven, 30~39 year old female. Ask Date: 2022/02/03

Dr. Hong Yuanbin reply Internal Medicine

Hello, "copy" and "copies" have the same meaning and refer to the PCR test index.
The equivalence of one copy to IU can vary depending on the instruments or reagents used, so it is necessary to consult the physician at the hospital to understand that the minimum of 33 IU means that the test can only detect a viral load of at least 33 IU.
If the viral load is below 33 IU, it will not be detectable and will be considered negative.
Thank you for your question.

Reply Date: 2022/02/03

More Info

To address your questions regarding HIV testing and the specifics of detection limits and measurement units, let's break down the information systematically.

Understanding HIV Detection Limits
1. Detection Limits: The detection limit of an HIV test refers to the lowest concentration of HIV that the test can reliably detect. In your case, the Roche Cobas 6800 System has a detection limit of 33 IU/mL. This means that if the viral load is below this threshold, the test may return a negative result even if the virus is present.

2. Units of Measurement: The confusion around the conversion between copies and International Units (IU) is common. Generally, the conversion factor can vary depending on the specific assay used. For the Roche Cobas system, 1 copy of HIV RNA is approximately equal to 1.67 IU. Therefore, if the detection limit is 33 IU/mL, this translates to about 20 copies/mL (since 33 IU/mL divided by 1.67 IU/copy equals approximately 19.76 copies/mL).
However, you mentioned various conversion factors you found (1 copy = 0.6 IU, 1 copy = 5 IU, etc.). These discrepancies arise from differences in assay methodologies and should be clarified with the specific laboratory performing the test. The Roche Cobas system is widely recognized for its accuracy, and the conversion factor of 1 copy = 1.67 IU is the one to rely on for your results.

3. Terminology: The term "copy" in the context of viral load testing refers to the number of viral RNA copies present in a milliliter of blood. "Copies" is simply the plural form of "copy." In this context, both terms are used interchangeably to describe the same measurement.

4. Minimum Detection Threshold: The stated detection limit of 33 IU/mL is indeed the minimum threshold for the Roche Cobas 6800 System. If your test result indicates no infection, it means that the viral load is below this threshold. However, it is important to note that while this test is highly sensitive, no test is infallible. If there are ongoing concerns or potential exposure to HIV, follow-up testing may be warranted.

Recommendations for Further Testing
If you have had potential exposure to HIV and your test results are negative, it is generally recommended to follow up with additional testing, especially if you have symptoms or if there was a high-risk exposure. The window period for HIV testing can vary depending on the type of test used:
- Antibody Tests: These tests can take 3 to 12 weeks to detect HIV.

- Antigen/Antibody Tests: These can typically detect HIV within 18 to 45 days after exposure.

- Nucleic Acid Tests (NAT): These tests can detect HIV earlier, usually within 10 to 33 days after exposure.

Conclusion
In summary, your test results indicating no HIV infection are reassuring, especially given the detection limit of 33 IU/mL. The conversion factor of 1 copy = 1.67 IU is applicable to your test, and both "copy" and "copies" refer to the same measurement. If you have any lingering concerns or symptoms, it is advisable to consult with a healthcare provider for further evaluation and possibly additional testing. Regular follow-ups and safe practices are key in managing your health and reducing the risk of HIV transmission.

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