Tissue Processing SOP (Template)

Purpose

To standardise the dehydration, clearing and paraffin infiltration of fixed tissue so that blocks section cleanly and slides stain consistently, with reproducible turnaround.

Scope

Applies to routine paraffin tissue processing of fixed histopathology specimens on the laboratory tissue processor. Excludes fixation (covered separately) and frozen-section work.

Responsibility

• Histotechnician — loads cassettes, selects the validated program, starts/monitors the run and records reagent changes.
• Senior technician / lab in-charge — approves schedules, authorises reagent rotation and reviews block quality.
• Pathologist / quality manager — approves this SOP and reviews recurring processing defects.

Equipment & materials

• Automatic tissue processor (vacuum/pressure-assisted or carousel) with a validated program set
• Graded alcohols (dehydration), clearing agent (xylene or xylene substitute), paraffin wax
• Labelled cassettes with adequately fixed tissue ≤ 3–4 mm thick
• Reagent-change log and processing register

Safety precautions

• Formalin, alcohols and clearing agents are hazardous — use in a ventilated/ducted area with PPE (gloves, coat, eye protection).
• Follow local biomedical-waste and solvent-disposal rules; keep a spill kit accessible.
• Confirm the processor lid/retort seals before starting a heated, solvent-laden cycle.

Procedure

Before the run

1. Confirm each specimen is adequately fixed before loading — under-fixed tissue will not process correctly. Minimum fixation time: [lab to validate] for your fixative and tissue types.
2. Verify tissue is grossed to ≤ 3–4 mm thickness; re-gross thick pieces rather than forcing a standard cycle.
3. Check reagent levels and position; confirm the reagent-rotation schedule is current.
4. Select the validated program for the load type (routine vs biopsy vs fatty/dense tissue). Cycle durations: [lab to validate].

Processing sequence

1. Dehydration through graded alcohols (ascending concentration) to remove water from tissue.
2. Clearing with xylene or a validated substitute to make tissue miscible with paraffin.
3. Paraffin infiltration under vacuum/pressure where available. Paraffin temperature: [lab to validate] (kept just above melting point; avoid overheating).
4. Run the program to completion; embed promptly after unloading to avoid over-hardening.

After the run

1. Unload cassettes and transfer to embedding without prolonged holding in hot paraffin.
2. Record run details, any alarms and reagent status in the processing register.

Quality-control checkpoints

• Blocks are firm and section cleanly without shattering or a wet/soft centre.
• Nuclei stain crisply on the resulting H&E slide (weak nuclear staining can indicate under-processing).
• No excessive hardness/brittleness or chatter traceable to over-processing.

Acceptance criteria

• Representative test block sections fully and stains with clear nuclear and cytoplasmic contrast.
• No recurring processing-related defects across the batch.

Common errors & corrective action

Records & documentation

• Processing register (date, load, program, operator, alarms)
• Reagent-change / rotation log
• Defect and corrective-action record

Review frequency

Review at least annually, or sooner after a reagent, equipment or protocol change, or a recurring defect.

Related equipment & guides