Overview
Biochemical identification uses controlled metabolic reactions (spot tests and multi-substrate panels) as evidence toward bacterial identification when integrated with Gram morphology and colony phenotype.
Classification
| Test | Typical branch point | Pitfall |
|---|---|---|
| Catalase | GPC grouping | Blood-agar carryover false (+) |
| Oxidase | GNR grouping | Reagent age / timing |
| Coagulase | S. aureus vs CoNS pathway | Mixed inoculum |
| Indole | Enteric pattern | Wrong broth / late read |
| Urease / citrate / TSI | Enteric pattern | Impure isolate |
Morphologic Features
Biochemistry never replaces Gram and colony morphology, reactions are interpreted only after the isolate’s microscopic and colonial hypothesis is stated.
Laboratory Characteristics
- Pure isolate, appropriate age
- QC organisms per method
- Manufacturer timing windows
- Panel/MALDI congruence checks
Reference Intervals
Biochemical “cutoffs” and panel acceptance scores are method- and version-specific, verify institutional instrument libraries and SOP acceptance rules.
Clinical and Laboratory Significance
Correct pathway use of biochemistry supports accurate organism naming for reports and AST handoff decisions based on identification, not MLS-directed therapy.
Differential Considerations
When reactions conflict, differentiate mixed culture, wrong test for Gram group, reagent failure, and true atypical phenotype before forcing an ID.
Comparison Tables
Evidence vs over-call
| Observation | Justified claim | Unjustified claim |
|---|---|---|
| Catalase (+) GPC clusters | Staph-group pathway open | S. aureus final |
| Oxidase (+) GNR | Non-Enterobacterales-leaning branch possible | P. aeruginosa final |
| Indole (+) lactose fermenter | E. coli among differentials | Species without full pattern |
Classification Frameworks
Not applicable.
Laboratory Notes
- Hedge institutional spot-test menus and panel databases
- Never report species from one spot test
- Purity check before expensive or multi-well panels
References
Authoritative textbooks, guidelines, and reviews supporting this reference entry. Verify reference intervals, critical limits, and reflex criteria against institutional protocols and current guideline editions.
Textbooks
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2020.
- Mahon CR, Lehman DC, Manuselis G. Textbook of Diagnostic Microbiology. 6th ed. Elsevier; 2018.
- Carroll KC, Hobden JA, Miller S, et al. Jawetz, Melnick, & Adelberg's Medical Microbiology. 28th ed. McGraw-Hill; 2019.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
CLSI and Professional Guidelines
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. CLSI document M100 (verify current edition and institutional adoption).
- Centers for Disease Control and Prevention / NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL). Current edition (verify institutional adoption).
Frequently Asked Questions
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