Around 18 members of public sector union Unison began a seven-day strike today at Mid Yorkshire, which runs Dewsbury, Pinderfields and Pontefract hospitals.
Unison said its members were being overworked because there are not enough medical laboratory assistants at the trust.
Jim Bell, Unison regional organiser, said: “The shortages are having a very bad effect on our members. Sickness levels are soaring and some are being forced to leave the job because of the enormous pressure they are under.
“Our members say they have been left with no alternative but to raise this dire situation in public through industrial action.”
Unison said it had agreed that emergency cover would be in place during the strike to ensure patients were not at risk.
Angela Wilkinson, director of human resources at Mid Yorkshire, said talks were being held with Unison.
She said: “Our number one priority is, as always, patient care and we will be putting in place a number of measures throughout the strike to ensure the laboratories continue to provide a service to all our patients.”