Temporary increases in HIV levels in patients under treatment generally do not mean the virus is developing resistance to drugs, US research finds.
Drugs must cope with a rapidly evolving virus
HIV mutates very rapidly, and doctors were worried that even small blips were a sign that the virus had found a way to combat sophisticated drug therapy.
But researchers told the Journal of the American Medical Association they were more likely to be a statistical quirk.
The Johns Hopkins University team had tested 10 HIV-positive patients' blood.
Concern about the blips have led to costly repeat testing, and in some cases alterations in therapy, which have now been shown probably to have been unnecessary.
HIV drug resistance
Drug resistance is a growing problem in HIV therapy
The virus can mutate faster than medical research can develop new drugs
When HIV becomes resistant to one drug, it may also become resistant to other drugs in the same class
There are only four classes of HAART drugs - a total of 20 drugs - available
Therefore the number of available combinations is limited
Lead researcher Dr Robert Siliciano said: "These results should provide relief to hundreds of thousands of HIV-positive patients currently taking drug therapy, called highly active anti-retroviral therapy, or HAART, and reassure them that their medications have not failed.
"Physicians and patients now have a much better idea of when to worry about these blips and when not to worry."
The researchers have calculated that unless the blip is higher than 200 copies per millilitre of blood, or persists upon repeated testing, it does not signal that the virus has mutated, and that the drugs are losing their effect.
True drug resistance requires changes in therapy which can be very difficult for the patient.
Different combinations of medications can have toxic side effects, such as diabetes, and can be considerably harder to tolerate than the originally prescribed drug cocktail.
The modern generation of anti-HIV drug treatments quickly suppress the virus to nearly undetectable levels - but blips are a frequent problem.
The Hopkins team conducted a detailed genetic analysis of multiple blood samples from 10 HIV-positive patients.
All patients had their infection under long-term control, on HAART, and with viral loads of less than 50 copies per millilitre for at least six months.
Blips occurred in nine of the 10 patients, but the results were not replicated when further tests were carried out on the same samples - giving weight to the theory that the fluctuations were simply down to statistics.
And the genetic analysis showed no evidence of mutations in either of the two key HIV enzymes which are blocked by drug therapy.
Yusef Azad, policy director at the National AIDS Trust, said: "Resistance to HIV drugs is a real cause for concern, due to the difficulty of adhering to strict drug regimens and the ability of the HIV virus to mutate.
"However this research shows that it is important not to jump to hasty conclusions based on slight changes in viral load."
Jo Robinson, of the HIV charity the Terrence Higgins Trust, said it was useful for patients to know that viral load blips appear to have no clinical significance.
Christopher Gadd, editor of HIV & Aids Treatments Directory, said: "This removes some of the uncertainty faced by doctors when their patient's viral load suddenly increases while on anti-HIV therapy.
"But although these results provide encouragement for patients, it's important that patients and healthcare workers remain aware of the risks of treatment failure."