Medical Plurals: Formation Rules, Common Irregulars, and Usage Guide

Medical terminology derives largely from Greek and Latin, resulting in pluralization patterns that differ from standard English. Correct plural forms are essential for precise clinical communication.

Regular Patterns (Latin/Greek Origin)

Singular Ending	Plural Ending	Examples
-us	-i	bronchus → bronchi, coccus → cocci, locus → loci, radius → radii
-a	-ae	vertebra → vertebrae, scapula → scapulae, formula → formulae
-um	-a	bacterium → bacteria, curriculum → curricula, datum → data, medium → media
-is	-es	diagnosis → diagnoses, prognosis → prognoses, thesis → theses, metastasis → metastases
-ex / -ix	-ices	appendix → appendices, matrix → matrices, index → indices, cortex → cortices
-on	-a	criterion → criteria, phenomenon → phenomena, ganglion → ganglia
-nx	-nges	phalanx → phalanges, larynx → larynges

Anglicized Plurals (Acceptable in Modern Usage)

• appendixes, formulas, memorandums, referendums, stadiums, terminuses
• Both forms often coexist (e.g., indexes/indices, appendixes/appendices); context determines preference

Greek-Specific Patterns

• -ma → -mata: schema → schemata, stigma → stigmata, sarcoma → sarcomata
• -sis → -ses: analysis → analyses, basis → bases, crisis → crises, thesis → theses
• -itis (diseases): usually unchanged in plural (multiple arthritides) or -itides

Common Irregulars (Must Memorize)

Singular	Plural
corpus	corpora
genus	genera
species	species
series	series
femur	femora
os	ossa
pons	pontes
foramen	foramina
meatus	meatus
viscus	viscera
diverticulum	diverticula

Clinical Importance

• Prescriptions: "Take 2 tablets" vs. "Take 2 tablet" – ambiguity risk
• Pathology reports: "Multiple adenomas" not "adenoma"
• Orders: "Cultures × 2" not "culture × 2"
• Research: "The data are" (plural) vs. "The datum is"

Quick Reference

• One bacterium, many bacteria
• One diagnosis, many diagnoses
• One criterion, many criteria
• One phenomenon, many phenomena
• One matrix, many matrices

When uncertain, consult a medical dictionary (Stedman's, Dorland's) or style guide (AMA Manual of Style).