ISO 20391-1:2018

Overview

ISO 20391-1:2018 (Biotechnology - Cell counting - Part 1: General guidance on cell counting methods) is an ISO international standard that defines terms and provides general guidance for cell counting in biotechnology. Published by ISO in 2018, the standard covers counting of cells in suspension (expressed as cell concentration) and cells adhered to a substrate (expressed as area density). It applies to all cell types - mammalian and non-mammalian (e.g., bacteria, yeast) - but is not intended for counting cells in tissue sections or biomaterial matrices.

Key topics and requirements

The standard focuses on practical and measurement-quality aspects of cell counting, including:

• Terminology and definitions - standardized terms (e.g., total cell count, differential count, direct vs indirect counting, limit of quantitation, reference material).
• Counting concepts - guidance on total and differential cell counting, and on direct (individual-event based) and indirect (population-signal based) methods.
• Method selection - considerations for choosing an appropriate cell counting technique for the intended measurand and application.
• Sampling and sample preparation - key factors affecting accuracy and precision (environmental factors, procedures, reagent quality/stability).
• Measurement process - performing measurements, instrument qualification, and procedural controls.
• Qualification, validation and verification - instrument qualification, method validation/verification, and use of reference materials (certified and in‑house).
• Data processing, analysis and reporting - guidance on image analysis, gating, coincidence correction, uncertainty, and consistent reporting.
• Informative annexes - descriptions of common cell counting methods and mapping methods to measurement purposes (Annex A and B).

Practical applications

ISO 20391-1:2018 supports reliable cell enumeration across biotechnology activities where accurate cell counts are critical:

• Biomanufacturing process control and in‑process monitoring
• Cell and gene therapy product potency and dosing
• Microbiology and fermentation (bacteria, yeast) concentration measurements
• Cytotoxicity assays and cell-based bioassays requiring normalization to cell number
• Quality control laboratories implementing validated cell counting workflows

Who should use this standard

• Laboratory managers and QA/QC personnel in biotech and pharmaceutical organizations
• Method developers, validation engineers, and instrument manufacturers (flow cytometers, automated counters, imaging systems)
• Regulatory scientists, assay designers, and researchers conducting quantitative cell biology assays

Related standards

The document notes that several sector- or application-specific international and national standards for cell counting exist; users should consult those when working within their specific technique or application scope.

Keywords: ISO 20391-1:2018, cell counting standard, cell concentration, area density, method validation, instrument qualification, cell counting methods, biotechnology measurement guidance.