A positive reaction/result: indicates a test or experiment detected the target (e.g., medical tests like PCR/antigen/pregnancy tests, or chemical assays).
PCR検査で陽性反応が出たため、医師の指示で自宅隔離となった。
Because the PCR test came back positive, he was instructed by the doctor to isolate at home.
妊娠検査薬に陽性反応が現れて、二人は喜んだ。
A positive result appeared on the pregnancy test, and the two were delighted.
化学実験で試薬の変化が見え、陽性反応と判断した。
A change in the reagent was observed in the chemical experiment, and it was judged a positive reaction.