Why is verb to 'be' omitted before 'tested' in the following sentence?
Almost a fifth of all chickens (18 per cent) tested positive for the bug above the highest level of contamination

Question

Answer

This is an intransitive meaning of "to test". From the Oxford Dictionary:
"[ no obj., with complement ] produce a specified result in a medical test, especially a drugs test or AIDS test: he tested positive for steroids during the race."

Answer

More simply, "they tested positive". No need for "be" here. Tested is the verb, past tense of to test.