Home tests for coronavirus should be available to NHS staff across the UK "very soon", according to the government's testing co-ordinator.
Prof John Newton acknowledged that health and care workers have struggled to access testing sites.
The government said lack of "demand" rather than capacity was behind the slow growth in testing numbers.
But the British Medical Association (BMA), the Royal College of Nursing (RCN) and Unison have challenged this.
They say long drives or difficulty accessing drive-through sites without a car were preventing staff from being tested.
There are 27 testing centres in total and there are reports of some staff having to drive hundreds of miles to reach their nearest site.
The government says there is capacity to do about 40,000 tests a day across the UK, but only about half - 20,000 tests - are actually being processed.
A Downing Street spokesperson said the government was "absolutely standing by" its target of carrying out 100,000 tests per day by the end of April.
The prospect of a home test offers some hope when it comes to another major barrier for staff: the test has to be done within the first few days of experiencing symptoms.
Some have been missing out because people have been too unwell to drive to a testing centre, according to Saffron Cordery, deputy chief executive of NHS Providers, which represents hospital trusts.
But overall, the proportion of staff who are eligible to be tested is actually quite small, she said.
At the moment, the priority is to test key workers who are off work either because they have symptoms or someone in their household does.
Ms Cordery estimated that roughly 150,000 staff are off at the moment, but about half of those will be suffering from other illnesses. Some will be shielding because of long-term conditions.
She said the rationale for the government's 100,000 tests a day target wasn't "entirely clear", but welcomed the "challenge" it provided.
Public Health England (PHE) is trialling a system of UK-wide self-testing for key workers, but until that's ready to roll out, most still can't be tested in their homes.
In the meantime, head of the Royal College of Nursing, Dame Donna Kinnair, said testing for staff and their households should take place "at or close to their workplace", to make sure it's accessible.
She said nurses had been telling the College that the existing testing sites were "located too far away from them".
While some doctors and nurses have been able to be tested at their hospitals, this hasn't generally been possible for care staff scattered across thousands of smaller sites.
In the case of those self-isolating because someone they live with has symptoms, it's the household members who need to be tested as well as the key worker.
This group will be larger and harder to reach since their referral will have to come via the health or care employee.
Follow Rachel on Twitter