Knowledge Base › Troubleshooting Atlas › Reagents & safety
Formalin (acid haematin) pigment
Also known as: formalin pigment, acid formaldehyde haematin
What it looks like
Fine dark brown-black birefringent granular pigment, typically over and around blood-rich areas.
Where it usually appears: Blood-rich or autopsy tissue fixed in unbuffered/acidic formalin.
Likely causes
- Acidic (unbuffered) formalin reacting with haemoglobin
- Prolonged fixation in unbuffered formalin
- Blood-rich specimens
Immediate checks
- Confirm you are using neutral buffered formalin
- Distinguish from melanin/malaria pigment under polarised light (formalin pigment is birefringent)
Corrective action
- Remove pigment with an alcoholic picric acid or alkaline-alcohol treatment if it interferes with interpretation
Preventive action
- Use 10% neutral buffered formalin
- Avoid prolonged fixation
- Ensure adequate buffering of fixative
This atlas is for educational purposes for laboratory professionals. It does not replace your institutional SOPs, manufacturer instructions, validated protocols or pathologist judgment. Adapt any action to your laboratory and equipment.
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