A term used to refer to brown pigment deposits on histological slides. It is better referred to as acid formaldehyde haematin or acid formol haematin (not to be confused with haematein, a component of haematoxylin) as it is not due to formalin itself (which is buffered formaldehyde with methanol), but rather a result of haemoglobin interacting with formaldehyde at an acidic pH. Formalin stored for prolonged periods may result in oxidation of methanal (formaldehyde) to methanoic acid (formic acid). Therefore this formalin pigment is more common:
- where formalin has been stored for a long time
- with bloody specimens
- with specimens stored for a long time, e.g. brain fixation
It may give rise to an artefactual appearence of pigment deposition, but should be distinguishable by its birefringence under polarised light. It is also tends to be extracellular.
Immersion in ethanolic picric acid or alkaline alcohol can remove the pigment. It is more likely to occur.