The state health department on Wednesday asked representatives of 30 hospitals and laboratories in the city not to perform the "rapid" test to detect the dengue virus as these examinations often throw up "false positive" results.

Department officials also asked the representatives at the Swasthya Bhavan meeting to scrap the "combo" pack that tests for both antigen and antibody of dengue, an "unscientific approach" according to doctors.

"The guidelines of the World Health Organisation and the National Vector Borne Diseases Control Programme say only the ELISA test is valid for the detection of dengue. But it has come to our notice that many laboratories in the city were using the rapid test. So we summoned representatives of all the laboratories that conduct dengue tests in the city and asked them to follow only the ELISA method," said Biswa Ranjan Satpathy, the director of health services.

Calcutta Municipal Corporation figures reveal that 165 dengue cases have been detected in the city so far this year. The corresponding figure last year was over 400.

For the entire state, the number of dengue cases so far this year is 928. The disease has claimed the life of one patient this year - a man from Nadia.

Rapid blood tests - also called "card" or "cassette" tests - are similar to a home pregnancy test. In this test a drop of blood is put on a slot in a card fitted with a reagent.

The ELISA test is much more elaborate and time-consuming and the results are authentic, said microbiologists.

"It is more profitable for laboratories to conduct rapid tests because that way they can conduct more tests. Also, only qualified technicians can conduct the ELISA test," microbiologist Satadal Das said.

At Wednesday's meeting, the laboratories and hospitals were also asked to scrap the practice of conducting combo tests, which cost between Rs 1,600 and Rs 1,800. The combo packs comprise the NS-1, iGm and iGg tests, which should be conducted at different time after the onset of the disease.

Satpathy said he had also requested the representatives to try and maintain a uniform rate for dengue tests.