Flow cytometry: weak signal

1. Ranked causes

   1. Antibody titre too low or conjugate lot inactivePositive-control cells still dim after titration; fresh vial restores MFI.
   2. Insufficient staining time or temperature for the epitopeLonger incubation on ice or at RT restores expected MFI.
   3. Fluorophore photobleached or stored incorrectlyConjugate left in light or freeze-thawed repeatedly.
   4. PMT / detector voltage set too low for the fluorophoreRaising voltage moves the positive population without changing stain.

2. Recovery steps

   1. Titrate the antibody on positive-control cells (e.g. 1:50 → 1:200)Separates under-titration from instrument settings in one plate.
   2. Raise PMT voltage using single-stained controls until positives clear the negativeUnder-voltage is a common silent change after a shared cytometer session.
   3. Extend staining time to the validated window (e.g. 30–45 min protected from light)Incomplete epitope engagement mimics a dead conjugate.

3. Controls first

   • Did the positive-control single-stained sample also show weak signal?If yes: Antibody lot, staining method, or detector settings — not this sample alone.
   • Does raising PMT voltage on the same stained tube recover separation?If yes: Instrument settings first — re-stain only if voltage alone is insufficient.

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