The HSE has warned of serious service delays from 8am to 8pm.
HSE National Director of Acute Services Liam Woods said the industrial action will cause “widespread disruption” as laboratory services are critical for the “effective” and “safe” running of hospitals.
“There are 14,000 outpatient attendances every day and the majority of those we think today will be cancelled,” he said.
“The details are on our website, HSE.ie, and we list those by hospital and by county and there is variation across the country to some extent… but the impact is significant.”
Mr Woods said there will be “significant cancellations” for surgeries, bar urgent cancer, transplant and “one or two other exceptions” will go ahead.
He added that people who have not been notified of a cancellation should still go to their appointment as planned.
Mr Woods also confirmed that today’s strike will put increased pressure on emergency departments and trolley numbers.
“There will be a delay in accessing services in emergency departments and indeed in moving from emergency departments up the hospital,” he said.
Members of the Medical Laboratory Scientists Association (MLSA) are taking the action in a long running dispute over pay and unfilled posts.
The medical scientists said they are taking the action “in frustration over long-standing pay and career development issues.”
The workplace of the majority of medical scientists is at laboratories in public hospitals.
The union representing Medical Scientists, the MLSA, said it has made every effort to avoid disruption to patients and fellow healthcare workers, but has been left with no alternative.
The action follows many rounds of unsuccessful talks with HSE, Department of Health, Department of Public Expenditure and Reform and the Public Service Agreement Group.
In a ballot of MLSA members last November 98pc voted in favour of taking the action.
If no progress is made a further two days of action are planned for May 24 and 25.
Three further days of action are planned for May 31, June and June 2.
MLSA chairperson Kevin O’Boyle said there is huge frustration and burn-out among medical scientists because of a severe recruitment and retention problem which have been ignored by the employer for many years and he set out the issues needing to be resolved.
“Up to 20pc of approved medical scientist posts are unfilled in hospitals,” he said.
Medical scientists carry out identical work to other colleagues in hospital laboratories, yet are paid on average 8pc less, he said.
And they have fewer career development opportunities and less support for training and education than comparable colleagues
The role for laboratory diagnostics is currently expanding with increasing responsibility and workloads.
“We need to achieve a sustainable work structure for the profession and this will benefit patients and the quality and efficiency of health services they receive.”
MLSA general secretary, Terry Casey, said the union is seeking meaningful talks with the HSE and Department of Health.
He said it is a long-standing dispute which the MLSA has made repeated efforts to resolve.
“The MLSA’s claim for parity with clinical biochemist colleagues dates back to 2001 when an expert group report recommended pay parity between the grades.
"The then awarded pay parity was lost within months as a result of an inadvertent procedural error in the first public service benchmarking awards in June 2002.
“In January 2020, against a backdrop of a severe and worsening recruitment and retention crisis, the MLSA renewed its longstanding claim for parity of pay and career progression with clinical biochemists and sought engagement with the HSE and Department of Health (DOH).
“More than two years on, and after many rounds of proposals and talks, these issues have not been resolved and there is now an even more significant nationwide shortage of medical scientists,” he added.