PCR / Gel: faint bands

1. Ranked causes

   1. Low template input (low-copy target or degraded RNA)Consistent faintness across replicates from the same extraction.
   2. Too few amplification cycles for template abundanceFaint but clean single band, no smear.
   3. Master-mix or polymerase lot past peak activity / near expiryYield dropped after a reagent lot change with the same protocol.
   4. Under-loading on the gelLoading dye is also faint relative to other lanes.

2. Recovery steps

   1. Increase cycle count by 3–5 cyclesEach extra cycle roughly doubles product in the exponential phase.
   2. Load 2× more PCR product per laneCheapest first check before touching chemistry.
   3. Re-quantify template input by Qubit/Nanodrop before the next runConfirms whether the issue is amplification efficiency or starting material.
   4. Swap to a fresh master-mix / polymerase aliquot within expiryRules out lot activity drop after storage or freeze-thaw.

3. Controls first

   • Did the positive control band look as strong as usual?If no: Reagent or cycle-count problem — not just this sample's loading.
   • Is the band single and clean (no smear)?If yes: Chemistry is working — this is a cycle-count or loading problem, not specificity.

Diagnose your run

Opens Diagnostics capture with this first-party playbook behind it. Sign in if the workspace asks.