Stories in several countries suggest people are having up to six negative results before finally being diagnosed.
Meanwhile, officials in the epicentre of the epidemic, Hubei province, China, have started counting people with symptoms rather than using the tests for final confirmation.
As a result, nearly 15,000 new cases were reported on a single day – a quarter of all cases in this epidemic.
What are these tests and is there a problem with them?
They work by looking for the genetic code of the virus.
A sample is taken from the patient. Then, in the laboratory, the virus’s genetic code (if it’s there) is extracted and repeatedly copied, making tiny quantities vast and detectable.
These “RT-PCR” tests, widely used in medicine to diagnose viruses such as HIV and influenza, are normally highly reliable.
“They are very robust tests generally, with a low false-positive and a low false-negative rate,” Dr Nathalie MacDermott, of King’s College London, says.
But are things going wrong?
A study in the journal Radiology showed five out of 167 patients tested negative for the disease despite lung scans showing they were ill. They then tested positive for the virus at a later date.
And there are numerous anecdotal accounts.
These include that of Dr Li Wenliang, who first raised concerns about the disease and has been hailed as a hero in China after dying from it.