The Canterbury District Health Board says a new strike notice targets some chronically-ill patients.
Laboratory workers at the DHB say they will not do tests for a couple of drugs used to monitor and treat patients with inflammatory bowel disease and arthritis.
The partial strike action is to take place from 3-9 November.
Canterbury Health Laboratories says the tests will be done, either by non-union staff or elsewhere.
However, it says that will not be as safe for patients, or as convenient as putting them through the routine laboratory system.
The laboratory's general manager, Trevor English, says the board's concern is that the union is targeting patients, although it has consistently said it is not doing so.
The Medical Laboratory Workers Union says only tests from outside the district are affected and they could be sent elsewhere.