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What Does ""Less Than Detectable Limit" S1 HIV-1 Ct - 26.10 S2 30.17" Mean In An PCR Test?

Posted on Thu, 30 Jan 2014

Question: I took a HIV Quantitative real-time PCR test 9 months after a infection causing event. The report states as follows: "Less than detectable limit" S1 HIV-1 Ct - 26.10 S2 30.17 In the graph plotted, there is a threshold and the S1 and S2 curves cross the threshold respectively at the above mentioned points... What does it mean? Should I be concerned?

Answered by Dr. Roopa Hiremath (43 minutes later)

Brief Answer: Real time PCR Detailed Answer: Hi Mr. XXXXXXX Thanks for choosing health care magic. I have gone through your query and I understand your concern regarding the test results of real time PCR. I want to elaborate on the procedure of real time PCR for you to understand the test results better. PCR is polymerase chain reaction which is an exponential amplification system which amplifies a specific target sequence and this target is detected as a graph on a computerized software in real time PCR. The target sequence for HIV tests is the HIV viral RNA or proviral DNA. Just detection of RNA does not suffice but a specific titre is significant which is defined as the threshold value. Values detected beyond threshold value in particular number of cycles is significant. Your test report is denoting that HIV nucleic acids is less then detectable limit and this test result is negative for HIV. I hope I have cleared your doubts. Please let me know if I can assist you further. Thanks

Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar

Follow up: Dr. Roopa Hiremath (12 minutes later)

Will the test result vary depending upon the number of days after a possible exposure? More specifically, I read somewhere that PCR test is effective very early (around 15 days from an exposure)... I want to clarify that the effectiveness does not go down with time (I tested after 9 months after an exposure)

Answered by Dr. Roopa Hiremath (45 minutes later)

Brief Answer: PCR is test of choice in early exposure Detailed Answer: Hi Mr. XXXXXXX Welcome back. Yes, you are right that PCR is test of choice in early exposure (within 3 months) because after 3 months, the person develops antibodies against HIV virus which try to kill the viruses and the viral load in blood may fall. But PCR detects as less as 10 nanograms of HIV RNA, so if HIV virus is present in the blood sample its RNA will be detected by PCR. If you are still in doubt, you can undergo an ELISA for both antigens and antibodies against HIV 1 and 2, which is test of choice in late exposure. This test will be to confirm your HIV negative status. I hope I have cleared your doubts. Please get back if any more clarifications. Thanks.

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Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar