Patients can expect "significant disruption" in hospitals today due to industrial action, the HSE has warned.
The strike involves 2,100 medical scientists, who are picketing over pay and career development issues.
The scientists mainly work in public hospitals and are refusing to carry out routine lab services from 8am to 8pm on Wednesday.
Liam Woods, the HSE's director of acute operations, said that up to 14,000 outpatient appointments will be cancelled because of the strike.
The one-day strike is likely to cause widespread disruption, he told RTÉ’s Morning Ireland. Much planned work will have to be deferred, patients have been contacted and their appointments will be rescheduled, he added.
Any patients who have not been contacted should assume that their procedure is going ahead and turn up for their appointment, advised Mr Woods.
Among the services impacted is testing at Emergency Departments. This will mean it will be slow to get into and out of EDs, he warned. It will also slow down transfer into the hospital system which will mean high trolley numbers.
GP services will also be impacted, he said, with no testing likely to go ahead on Wednesday.
Mr Woods advised patients to check out the HSE website which lists what testing services are available on a county by county and hospital by hospital basis.
While planned surgeries will be cancelled, vital procedures for cancer treatment will go ahead, as will dialysis, said Mr Woods.
There is a threat of a further two days of strike action next week which will have a significant impact, he warned. The HSE is available to talk to resolve the issue, said Mr Woods.
Five more days of industrial action are planned for the coming weeks – on May 24th, May 25th, May 30th, June 1st and June 2nd.
Terry Casey, the general secretary of the Medical Laboratory Scientists Association, said it was important that the further strikes do not go ahead.
"I think we need to ensure that those actions don't proceed, and the only way that can happen is by really sitting down now and try to address the issues to a finality. And unfortunately that has been missing to date."