ASCP HTL key terms

The process of preserving cells and tissue structure by chemical or physical means so the specimen resists autolysis and putrefaction and survives processing, storage, and staining.

A fixative of 4% formaldehyde in phosphate buffer at pH 7.2-7.4; it is the routine standard because it penetrates well and preserves morphology for most stains.

A cross-link formed when formaldehyde reacts with amino groups on adjacent proteins, which is the main way formalin hardens tissue and preserves its structure.

The interval between loss of blood supply to a tissue and the beginning of fixation; long warm ischemia allows enzymatic degradation that damages morphology and nucleic acids.

The processing step in which graded alcohols progressively replace water in the specimen, preparing it for the clearing agent that follows.

Replacement of alcohol with a solvent such as xylene that mixes with both alcohol and paraffin, making tissue translucent and ready for infiltration.

Placing an infiltrated specimen in a mold, surrounding it with paraffin, and orienting the surface to be cut toward the block face.

Cutting thin, uniform sections from a paraffin block with a microtome, usually 3-5 micrometers thick for routine hematoxylin and eosin slides.

Removal of calcium from bone and other calcified tissue with acid or chelating agents so it can be sectioned; excessive treatment impairs staining and nucleic acid quality.

A natural basic dye from logwood that, once oxidized to hematein and bound to a metal mordant, stains chromatin and other acidic structures blue-purple.

An acidic anionic dye that stains cytoplasm, collagen, and erythrocytes shades of pink to orange-red and serves as the counterstain in routine hematoxylin and eosin preparations.

A step after hematoxylin staining in which a mildly alkaline solution raises the pH and turns the dye a permanent blue instead of remaining reddish-purple.

Selective removal of excess dye from overstained sections using an acid alcohol or other differentiating agent, leaving only the target structures colored as intended.

A metal ion, commonly alum, that links a dye molecule to tissue components and renders the dye-tissue complex insoluble; hematoxylin needs a mordant to stain nuclei.

A method in which periodic acid oxidizes tissue carbohydrates to aldehydes that react with Schiff reagent, producing magenta staining of glycogen, mucins, fungi, and basement membranes.

Enzyme pretreatment that digests glycogen before PAS staining; if the magenta staining disappears after digestion, the material was glycogen rather than mucin.

A three-dye connective tissue method that colors collagen blue or green while muscle, cytoplasm, and red cells appear red, used to assess fibrosis and architectural change.

A dye that binds amyloid proteins and produces apple-green birefringence under polarized light; it is the standard confirmatory stain for amyloid deposits in tissue.

A method in which ferric iron in tissue reacts with dilute hydrochloric acid and potassium ferrocyanide to form insoluble blue pigment, demonstrating hemosiderin and other iron deposits.

A stain in which carbolfuchsin with heat or phenol drives dye into the waxy walls of mycobacteria, which resist decolorization by acid-alcohol and appear red against a counterstained background.

The precise site on an antigen molecule to which an antibody binds; formalin fixation can mask epitopes, so retrieval steps are often required before staining.

A pretreatment, typically heating slides in buffer (heat-induced) or applying enzymes, that reverses formalin cross-linking and exposes epitopes so primary antibodies can bind.

A detection method using a polymer backbone carrying many secondary antibodies and enzyme molecules; it amplifies signal with less nonspecific background than older avidin-biotin methods.

3,3'-Diaminobenzidine, a chromogen that peroxidase oxidizes into a brown, insoluble precipitate at the site of antibody binding, giving permanent, alcohol-resistant staining.

Incubation with hydrogen peroxide before applying the primary antibody, which inactivates naturally occurring tissue peroxidases that would otherwise produce false-positive brown staining.

A control using an antibody of the same class, host, and concentration but unrelated specificity, showing whether staining reflects the target antigen rather than the immunoglobulin itself.

A method that demonstrates enzyme activity by incubating sections with a substrate that forms a colored or insoluble product exactly where the enzyme is located.

An instrument that transfers specimens through graded alcohols, clearing agent, and molten paraffin on a timed, often vacuum-assisted schedule to complete dehydration and infiltration.

A programmable instrument that delivers hematoxylin, eosin, wash, and differentiating solutions to racks of slides, improving consistency, throughput, and turnaround compared with manual staining.

Water purified and monitored to meet CLSI specifications for resistivity, organic content, and microbial counts, used for preparing reagents, stains, and buffers so impurities do not affect results.

OSHA's permissible exposure limits for formaldehyde: an action level of 0.5 ppm as an 8-hour time-weighted average and a 15-minute short-term exposure limit of 2 ppm.

A standardized document accompanying hazardous chemicals that describes hazards, composition, safe handling, exposure controls, and first aid, required under OSHA's Hazard Communication Standard.

An external program that sends unknown specimens to a laboratory and compares its results with peer laboratories; participation is required for accreditation and demonstrates analytical accuracy.

An intraoperative technique in which fresh tissue is frozen, sectioned, and stained quickly so a pathologist can give a diagnosis while the patient is still in surgery.

A refrigerated microtome, usually held near minus 20 degrees Celsius, that cuts thin sections of fresh frozen tissue for rapid diagnosis or for enzyme and lipid studies.

A technique in which fluorescent DNA probes hybridize to complementary sequences in nuclei, allowing detection of gene amplification, deletion, and translocation without loss of morphology.

A collection method in which cells are rinsed into preservative fluid so a thin, even monolayer can be prepared, reducing obscuring blood, mucus, and air-drying artifact.

A cytology preparation made by embedding centrifuged cell sediment in paraffin so sections can be cut and used for special stains and immunocytochemistry.