It comes as the health board urgently accelerated vaccinations in the local area amid rising case numbers and an “uncontrolled” outbreak.
Positive cases per population are higher in Moray than anywhere else in the country, reaching 95 per 100,000 at the end of last week.
NHS Grampian issued a statement to combat “rumours and hearsay” that the rise in cases had been caused by false test results.
"We can't quite believe we have to say this, but here goes: rising case numbers in Moray are not the result of people 'fooling' lateral flow devices with tomato ketchup (or any other condiment for that matter),” the health board said in a statement on Twitter.
"A positive lateral flow device test is always followed up with a PCR test to confirm the result. This is the gold standard of testing. 90 per cent of the positive LFD tests in Moray have been confirmed as positive by PCR testing.
"Please set aside the rumours and hearsay. The situation in Moray is real and it is serious. We are seeing people requiring hospital treatment as a result of Covid-19 infection. We need people to #ProtectMoray, stick to the guidelines, and take up the expanded testing offer.
“To everyone in Aberdeenshire and Aberdeen City, remember this virus can go wherever it wants and certainly doesn't respect local authority boundaries. Take care of yourselves, take care of your community, and let's keep going.”
The rumour that tomato ketchup can “test positive” for Covid-19 has previously spread on social media, with experts labelling it as false because condiments and other substances cause a test to malfunction rather than indicating presence of the virus.
NHS Grampian said that as positive lateral flow tests are confirmed by a gold-standard PCR test, the rumour could be discounted.
Dr Alexander Edwards, associate professor in Biomedical Technology at Reading University, told the FullFact website on the matter: “If you completely ignore the manufacturer’s instructions or in fact use the test for something completely different, then you shouldn’t really be surprised if you get a silly result.”