Are BMC figures telling half the story regarding dengue cases?

MUMBAI: The BMC's statistics on dengue cases in the city may have shown a slight increase, but figures from private diagnostic centres paint a scarier picture. If the data from private laboratories is to be believed, then more than a thousand Mumbaikars tested positive for dengue last month  and the number is likely to double this month.
Metropolis Healthcare, for instance, recorded 481 dengue cases from the start of the month till Wednesday. The BMC, during the same period, has registered 143 cases. In September, 698 tested positive for dengue at the private laboratory, whereas the BMC recorded 242 positives.
It seems, the dengue situation in the city is graver the BMC thinks it is.

One reason, say experts, is that there is no uniformity in the diagnosis procedure of dengue. Dr Jayanti Shastri, head of microbiology department at civic-run Nair Hospital, said they are seeing two-three dengue positives everyday. "PCR test is done in the initial phase and is free of cost for civic patients. The antibody tests have limited value and certain kits detect both antigen and antibodies. But the problem is that diagnostic centers across the city use various test kits, which may give false positives or false negatives," she said., adding that this makes it impossible to get a fair picture of the disease load in the city.
An antibody is a large Y-shaped protein produced by B-cells that is used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. An antigen is a substance that evokes the production of one or more antibodies. Thus, the presence of particular antibodies or antigens can prove that a person suffers from dengue.
The Metropolis data suggests that more people may be going for testing to a private set-up, which is why the number of positives is high too. "Most of the times, a patient is sent for antibody and antigen test in our lab. But the percentage of those testing positive for both at the same time is very low," said Dr Rajesh Bendre, consultant pathologist and head of immunochemistry department at the lab.
Dr Simi Bhatia, lab director, SRL Diagnostics, said the antigen and antibody tests are available as rapid tests as well as ELISA tests. "One test (IgM) detects better between Day 3 and Day 10 of infection, while the other (IgG) can detect better after the first week of the infection. Antigen is detected in patients with both primary and secondary dengue infections from Day 1 up to nine days after the onset of illness. Thus, the combined detection of NS1 antigen, IgG and IgM antibodies in single step helps for early and differential diagnosis of primary and secondary dengue infection."