It's possible the form listed the drugs she was on, but the social worker didn't know it was their job to figure out which results to ignore.

I've literally seen a Texas judge - who not only presumably court ordered drug tests regularly, but was also an ex-nurse - not understand how drug tests work. She assumed the lab would eliminate prescription-caused positives from the results. It took subpoenaing the tech who administered the test - a person in the same courthouse - to take the stand and tell the judge "we just list what the test found and what meds the person said they were taking, it's someone else's job to cross reference the two" before the judge stopped assuming the person on prescription Adderall was a meth head.

If an ex-nurse who deals with drug tests on a nearly daily basis doesn't understand how they work, I wouldn't be surprised at all if it turned out that a social worker misinterpreted the results similarly.