Testing a spot of dried blood on a piece of paper could help monitor HIV patients in developing countries.
The test, developed by researchers from University College London, could replace existing expensive methods.
Using the filter paper test would make it easier for doctors to monitor patients on anti-retro viral therapies.
The researchers, writing in the Lancet, said test results compared well with results from standard tests where a blood sample is tested.
Doctors need to be able to measure CD4 and lymphocyte counts to assess how patients are responding to treatment.
Low CD4 counts show a patient needs intensive treatment.
But conventional testing, where blood samples have to be taken, stored, and sent off to laboratories for testing is of limited value in countries where patients can live a long way from testing facilities.
Scientists have now developed a simple enzyme-based procedure where dried blood spots on filter paper are analysed.
Doctors from University College London, UK, and the University Teaching Hospital, Lusaka, Zambia, obtained blood from 42 HIV-infected patients.
Blood spots were dried on filter paper and CD4+ lymphocyte counts were measured.
These were compared with measurements from the usual test, the standard flow cytometry-the laboratory, the 'gold standard' for CD4 counts.
The paper test was able to accurately measure CD4 counts.
Dr Alimuddin Zumla of University College London, said: "Many African countries are now introducing antiretroviral therapy to their HIV-infected populations.
"The monitoring of this treatment requires CD4 count measurement which unfortunately is currently not available to the majority of poor people living away from cities where the health clinics and hospitals are based.
"Furthermore, the current method of testing is expensive."
He added: "Our results are very encouraging as they point the way towards making cheap CD4 count testing easily available to people receiving antiretroviral therapy in rural areas.
"Such methods could be used in a similar way towards HIV viral-load measurements, another test required to assess the success of HIV treatment."
'No storage needs'
Keith Alcorn, of the National Aids Manual told BBC News Online said the research was "potentially interesting".
But he said many patients in developing countries did not see doctors until their disease was well advanced - and their CD4 counts were much lower than the levels the paper test could measure.
"Until they can measure people with lower counts, it may have limited usefulness.
"But if they can sort that out, so it is more reliable in people with more advanced disease, then it will represent a very important step, because the blood won't have to be stored and it will be easier to transport the dried blood to the lab."
The research is published in The Lancet.