Cell Culture: clumping

1. Ranked causes

   1. Incomplete trypsinization / cells not fully dissociated before platingClumps visible immediately at seeding, before any growth.
   2. Insufficient trituration (pipetting) after dissociationClumps break up easily with additional pipetting of the same suspension.
   3. Overgrown culture prior to passageSource flask was over-confluent at harvest.
   4. Divalent cation (Ca²⁺/Mg²⁺) contamination in the dissociation bufferEDTA/trypsin buffer was not made cation-free.

2. Recovery steps

   1. Extend trypsin/Accutase incubation by 1–2 min and confirm single-cell suspension under the scope before platingDirectly fixes incomplete dissociation, the leading cause.
   2. Triturate 10–15× with a P1000 through a narrow tip, avoid bubblesMechanically breaks residual clumps after enzymatic dissociation.
   3. Pass the suspension through a 40 µm cell strainer before counting/platingRemoves remaining aggregates without affecting viable single cells.
   4. Prepare dissociation buffer in Ca²⁺/Mg²⁺-free PBSDivalent cations support cell-cell adhesion and work against dissociation.

3. Controls first

   • Did parallel control flasks of the same line also clump under the same dissociation protocol?If yes: Dissociation method or buffer composition — not one flask's contamination alone.
   • Do clumps break apart with extra pipetting right after harvest?If yes: Mechanical/trituration issue — not a media or contamination problem.

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