Diagnostics firms testing for coronavirus are nearing breaking point in cities like New Delhi and Mumbai as India battles its biggest surge in Covid-19, which may worsen the crisis as many sick people can't get tested fast enough to isolate themselves.

"We can't cater to the demand," said Dr. Vidur Mahajan of Mahajan Imaging in the Indian capital, who has temporarily shut two of his three sample collection points due to a backlog of pre-booked tests, and to prioritise testing for government hospitals.

India has recorded more than 200,000 new Covid-19 cases daily for the last six days, marking the world's worst jump this month. But many people have failed to get tested at all or early, which means the reported caseload of 15.3 million may be much lower than the real extent of the infection.

Officials from four diagnostics companies, including Mahajan, said samples currently being tested daily were between 300%-650% higher than February, putting infrastructure and personnel under severe pressure.

Doctors and patients in New Delhi and financial hub Mumbai said it was taking between three and eight days to find slots for the highly accurate RT-PCR tests and get their results.

If sample arrivals rise further, by 25% to 30%, "probably the testing facilities will crash, in terms of turn-around time at least," said A. Ganesan, group vice chairman of Neuberg Diagnostics, which runs 14 testing labs across India.

"We will have to turn back some of the patients without collecting their samples."

India's health ministry did not immediately respond to questions from Reuters. The government said 1.5 million samples were tested on Monday, a jump from about 1 million at the start of the month.

With hospitals full and oxygen and medicines in short supply, several cities are reporting far larger numbers of cremations and burials under coronavirus protocols than official Covid-19 death tolls, according to crematorium and cemetery workers, media and a review of government data.

New Delhi resident Puneet Vig said it took several calls to four labs this week before he was able to book a test for his 62-year-old mother who has been running a fever.

Her sample is due to be collected on April 24, with at least a couple of more days of delay expected till her results become available, he said.

"If you can't get a basic thing like testing done even in Delhi, it is very frustrating," Vig said.

With test results often delayed, doctors said they were relying on CT scans and symptoms to treat patients.

"What is happening because of the delayed testing is that the circle of transmission is getting wider and wider," said Dr. Ravindra Khade Patil of Sushrut Hospitals on the outskirts of Mumbai.

"Without a test, the patient may not isolate and thus infect others."