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Diff-Quik is a commercial Romanowsky stain variant, commonly used in cytological staining to rapidly stain and differentiate a variety of smears, commonly blood and non-gynecological smears, including those of fine needle aspirates. It is based on a modification of the Wright Giemsa stain pioneered by Bernard Witlin in 1970. It has advantages over the older Wright Giemsa staining technique, as it reduces the 4-minute process into a simplified 15-second operation, and allows for selective increased eosinophilic or basophilic staining depending upon the time the smear is left in the staining solutions.
Diff-Quik is utilized on material which is air-dried prior to alcohol fixation (rather than immersed immediately i.e. "wet-fixed").
The primary use of Romanowsky-type stains is for cytoplasmic detail, such as intracytoplasmic mucins, fat droplets and neurosecretory granules. Extracellular substances, such as free mucin, colloid, ground substance, etc., are also easily stained, and appear metachromatic. Microbiologic agents, such as bacteria and fungi, also appear more easily in Diff-Quik. 
There are generic brands of such stain, and the trade name is sometimes used loosely to refer to any such stain (much as "Coke" or "Band-Aid" are sometimes used imprecisely). The misspelling "Diff-Quick" is commonly encountered.
The Diff-Quik stain consists of 3 solutions:
- Diff-Quik fixative reagent
- Diff-Quik solution I (eosinophilic)
- Diff-Quik solution II (basophilic)
- Thiazine dye
- pH buffer
- Demay: The Art and Science of Cytopathology
|Neutrophils||Blue nucleus, pink cytoplasm, violet granules|
|Eosinophils||Blue nucleus, blue cytoplasm, red granules|
|Basophils||Purple/dark blue nucleus, violet granules|
|Monocyte||Violet nucleus, light blue cytoplasm|
|Spermatozoa||Pale blue in the acrosomal region and dark blue in the post-acrosomal region|