mymedschool.orgM08.01.001 Diagnostic techniques

Pathologic examination of tissues and organs is a two-step process used to identify:

The organ
The pathologic abnormality (e.g., cirrhotic nodules in the liver, necrotic tumor in the lung)

Both gross (macroscopic) and microscopic features play crucial roles in diagnosis.

Gross Examination

Purpose
1. To observe visible characteristics that suggest disease.
2. Important Gross Features
  - Size: Enlargement or atrophy (e.g., cardiomegaly, renal atrophy)
  - Shape: An altered contour may suggest neoplasia or fibrosis
  - Consistency: Hard (fibrosis/tumor), soft (necrosis), cystic, etc.
  - Color: Pale (ischemia), yellow (fatty change), dark (hemorrhage, congestion)
3. Next Step
  - Representative tissue sections are taken for microscopic examination, which provides the definitive diagnosis.

Example:

Microscopic evaluation is performed on stained tissue sections mounted on glass slides.
A single specimen may require multiple stains for accurate diagnosis.

Histochemical Stains
1. Use chemical reactions to highlight specific structures or organisms.
Hematoxylin and Eosin (H&E) Stain
1. Gold standard for histological evaluation.

Component	Color	Structures Stained
Hematoxylin	Blue-purple	Nuclei, nucleic acids, calcium salts
Eosin	Pink	Cytoplasm, extracellular & intracellular proteins

Note: The term “eosinophilic” refers to structures or cells that stain pink with eosin.

Use antibodies tagged with color to identify specific cell types or antigens.
(Positive staining = dark brown color)

Antibody Marker	Target Structure / Cells	Diagnostic Use
Cytokeratin	Epithelial cells	Carcinomas
Vimentin	Mesenchymal cells (except muscle)	Sarcomas
Desmin	Muscle (smooth, skeletal, cardiac)	Myogenic tumors
PSA	Prostate tissue	Prostate carcinoma

Note: Reticulin and Trichrome stains help identify fibrosis and nodules in cirrhotic liver.

Method	Use / Application
Microbiology	Identify bacteria, fungi, and viruses
Immunofluorescence Microscopy (IFM)	Detect immune complexes (e.g., renal & autoimmune diseases)
Electron Microscopy (EM)	Detect ultrastructural changes in renal, neoplastic, infectious, or genetic disorders

Applied to fresh or formalin-fixed tissue for genetic and molecular diagnosis.

Technique	Principle / Use
Protein Electrophoresis	Detect abnormal serum or urine proteins
Southern Blot	DNA analysis
Western Blot	Protein analysis
Polymerase Chain Reaction (PCR)	Amplifies specific DNA sequences
Next Generation Sequencing (NGS)	Detects mutations, chromosomal abnormalities, and gene expression patterns

Etiology vs Pathogenesis vs Clinical Significance

Term	Definition
Etiology	Cause of disease (genetic or environmental)
Pathogenesis	Sequence of events leading from cause to cellular injury and disease
Clinical Significance	Manifestations (signs/symptoms), complications, prognosis

The gross exam assesses size, shape, consistency, and color.
A microscopic examination confirms the diagnosis using specialized stains and techniques.
H&E is the gold standard for tissue evaluation.
IHC stains help classify tumors by cell origin.
Molecular and ancillary techniques enhance diagnostic precision.

By the end of this topic, the medical student should be able to:

Describe the steps and purpose of gross and microscopic tissue examination.
Identify common histologic stains and their diagnostic applications.
Recognize the role of immunohistochemistry and molecular methods in modern pathology.
Differentiate etiology, pathogenesis, and clinical significance of disease processes.