USP <855> Nephelometry and Turbidimetry

USP General Chapter <855> provides the compendial framework for measuring turbidity in liquid samples using nephelometry and turbidimetry. It is a General Chapter that defines standardized methodology. Specific acceptance criteria are established in individual monographs or validated product specifications. 

Note: USP General Chapters numbered below 1000 (such as <855>) are enforceable when referenced. Chapters numbered above 1000 are informational.

The methods are applied when a USP–NF monograph or general chapter explicitly specifies nephelometric or turbidimetric measurement, or when selected as part of a validated QC procedure.

In practice, USP <855> often raises questions around method selection, calibration strategy, and how turbidity data should be positioned relative to clarity and particulate requirements. These practical considerations are not addressed directly in the chapter.

Applicability
Applicable to USP–NF tests that specify or justify nephelometric or turbidimetric measurement of turbidity. Commonly used as a supporting measurement for parenteral clarity assessment, raw material quality control, and certain microbiological assays.

• Core Principles

  • Light scattering occurs when suspended particles disrupt the path of incident light.

  • Turbidimetry: measures the decrease in transmitted light intensity along the incident beam (180° geometry). Increased turbidity decreases transmitted intensity.

  • Nephelometry: measures light scattered at an angle (commonly 90°) to the incident beam. Increased turbidity increases scattered intensity.

  • Nephelometry is generally more sensitive for low turbidity; turbidimetry is suited for moderate to high turbidity.

Apparatus

Turbidimeter or nephelometer with stable light source, sample cell, and detector. Detector placement differs: 180° for turbidimetry, 90° for nephelometry. Cells must be optically clear and free from contamination.

Calibration & Standards

Formazin polymer suspension is the primary standard (stock solution: 4000 NTU, diluted to prepare working standards).

Formazin preparation involves hydrazine sulfate, which requires appropriate safety controls, and working dilutions have limited stability—factors that drive many labs toward commercial stabilized alternatives.
When commercial standards are used, equivalence to Formazin must be demonstrated and documented.
Results are commonly expressed in Nephelometric Turbidity Units (NTU). Turbidimetry may also be reported as absorbance, provided the method is calibrated against Formazin standards.

Operational Considerations

Avoid measurement artifacts from dirty cuvettes, fingerprints, air bubbles, dust, or unstable suspensions. Perform measurements promptly after sample preparation. Verify instrument sensitivity and linearity over the turbidity range relevant to the intended application, particularly for low-turbidity parenteral samples.

Relationship to <790> and <788>

Turbidity data may support investigations or trending but are not a substitute for compendial particulate or clarity tests unless explicitly justified and accepted.

USP <855>  In a CMC context, particularly for biologics, instrumental turbidity measurements under USP <855> are commonly used to quantitatively assess opalescence or clarity when visual inspection alone lacks sensitivity or trending capability. In such cases, turbidity may be established as a reportable or specification-linked attribute through product-specific validation and regulatory acceptance. However, instrumental turbidity does not eliminate the requirement for compendial visual inspection under USP <790>.

USP <790> addresses visible particulates through visual inspection.

USP <788> addresses subvisible particulate matter by particle counting methods.

These chapters serve different purposes. If your clarity assessment uses instrumental turbidity measurement, <855> defines how—but it does not replace the requirements of <790> or <788>.