Reference Standards provided by the United States Pharmacopeial Convention (USP Reference Standards, or RS) are highly characterized specimens reflective of specified drugs and foods (drug substances, biologics, excipients, dietary supplements, food ingredients, impurities, degradation products, reagents, and performance verification standards). When approved as suitable for use as comparison standards for documentary tests or assays (i.e., as a monograph component) in the United States Pharmacopeia (USP) or National Formulary (NF), USP RS also assume official status and legal recognition in the United States. Assessment of the suitability for use in other applications rests with the user. Official USP RS are primary standards in jurisdictions that so recognize them as such and, when appropriate, are calibrated relative to international reference materials such as those provided by the World Health Organization. USP RS are never intended for therapeutic use. USP's RS are provided for legal metrology purposes and can help ensure comparability of results and traceability to Système International d'Unités (SI) units whether certified or not. USP RS are Reference Materials as defined in the International Vocabulary of Metrology—Basic and General Concepts and Associated Terms (VIM): 3rd Edition 2007.

Reference Standards for USP or NF Articles
Reference Standards for official articles in USP or NF are provided as pure materials or as mixtures of chemicals reflective of the corresponding drug substances or excipients. The use of these materials is specified in the article's monograph, and these materials generally are necessary for use in the Assay and/or the Identification tests. The suitability of a USP RS for uses outside those specified in a monograph is the responsibility of the user. The property value or calculation value of the Reference Standard is stated on the label and should be included in calculations used in the monograph and applicable general chapters. For Reference Standards that do not bear a property value or calculation value on the label or in accompanying documentation, assume the Reference Standard is 100.0% pure for compendial quantitative applications.
Impurity Reference Standards
Reference Standards for impurities may include the following:
  • Organic impurities that may arise either during the manufacturing process or during the shelf-life storage of an article and may include starting materials, intermediates, by-products, reagents, catalysts, and/or degradation products.
  • Inorganic impurities that normally result from a synthesis process and may include reagents, catalysts, heavy metals, or inorganic salts
  • Residual solvents that may be either inorganic or organic liquids that are used to prepare solutions or suspensions during the synthesis of an article
Impurity Reference Standards may be presented as purified single-component materials or as mixtures of more than one impurity. Other options for controlling impurities may include presenting the official article with a labeled impurity content; using relative chromatographic retention times and response factors; or providing theoretical values such as UV absorptivities at selected wavelengths.
In earlier editions of the compendium, impurities were designated by their chemical names. For ease of indexing and searching, these have been gradually replaced with the designation “X related compound Y RS”, where X is the name of the official article, and Y is a sequential alphabetical letter. The assignment of this letter does not necessarily match the naming schemes of other compendia. Reference Standard impurity mixtures may also be designated by their intended use, such as “X System Suitability RS”. The conventional names and the chemical names are reproduced in the catalog and on the RS product label.
Certified Reference Materials
USP's Certified Reference Materials (CRMs) are Reference Standards that provide certified property values with associated uncertainties and metrological traceability, in accordance with International Organization for Standardization (ISO) Guides 30–35. Correct use of these CRMs support traceability of results to SI units and comparability of procedures.
USP Reference Standards for Biologicals
USP provides RS for biologic drugs and ancillary materials. For historical and other reasons, and as noted in Section 5.50.10 Units of Potency (Biological) in the General Notices and Requirements, USP RS for biologicals may diverge in unitage, by definition, or otherwise from other internationally recognized standards. Unless so noted in the documentary standard, international reference standards generally are not interchangeable and the USP RS is required in the tests and assays of USP–NF.
NF Reference Standards
Reference Standards currently labeled as “NF Reference Standards” are intended to be designated and labeled as “USP Reference Standards” pursuant to the consolidation of USP and NF within the USP as of January 2, 1975. Where a USP Reference Standard is called for, the corresponding substance labeled as an “NF Reference Standard” may be used.
Transition of Authentic Substances to USP Reference Standards
Previously, highly characterized reference materials not required for use in a USP–NF monograph or general chapter were developed by USP as a service and were distributed as Authentic Substances (AS). AS typically are highly characterized chemicals that are collaboratively tested and made available as a service primarily to analytical, clinical, pharmaceutical, and research laboratories. Such materials may be used for identification, method development, evaluation of method performance, or other applications as found suitable and validated by the user. USP will no longer introduce materials labeled “Authentic Substances.” All reference materials released, whether or not required for use in a USP–NF monograph or general chapter, will be “USP Reference Standards.”
Authentic Visual References
Authentic Visual References are USP Reference Standards, but unlike chemical reference materials, Authentic Visual References (AVR) are not used in chemical analyses. Instead, AVR are visual images used by analysts to compare certain test articles to ensure that they meet compendial requirements. AVR are incorporated by reference into the monograph.
USP Performance Verification Test Standards
These materials are provided to analyze and where appropriate to facilitate adjustment of the operation of an instrument to ensure that the results obtained are accurate and/or precise or otherwise give acceptable results. The use of these Reference Standards is generally described in associated general test chapters and allied information.

Official applications of USP RS are specified in USP–NF monographs and general chapters. They include the following:
  • quantitative uses in assays for drug substances and formulations, limit tests, or blanks and controls
  • qualitative uses, (e.g., identification tests, system suitability tests, or chromatographic peak markers)
  • method-specific uses, (e.g., performance verification standards, AVR, melting point standards, and the particle count set)
As described above, USP also provides Authentic Substances, not specified for use in a USP monograph or general chapter, which are used at the user's discretion.

The amount of material per individual USP RS container depends on the compendial application of the standard and is generally sufficient for several replicates. Some standards (mainly materials with significant handling requirements or materials that are available only in small amounts) are provided in single-use containers. Such single-use products generally are lyophilized, and their content is labeled in mass or activity units per container. If so labeled, the content of the container should be reconstituted in its entirety without any additional weighing. Instructions for reconstitution are given either on the label or in the monographs where the standard is used.

The label text provides all the information needed for the correct storage and use of the USP RS in monograph applications. The label includes directions for use, safety warnings, required information for controlled substances, and a property value or calculation value for standards with quantitative applications. For performance verification standards, acceptance ranges are provided. Where necessary, USP RS are accompanied by additional documentation such as Technical Data Sheets or Typical Chromatograms.
Unless otherwise directed in the procedure in the individual monograph or in a general chapter, USP RS should be used in accordance with the instructions on the label of the Reference Standard. Material Safety Data Sheets for all USP reference materials are available on the USP Web site.
Although USP RS undergo retesting on a predefined schedule to determine continued suitability for use, USP RS do not carry an expiration date on the label. A lot of USP RS may be used in its official applications as long as it is listed as “Current Lot” in the current USP Reference Standards Catalog or has not reached its Valid Use Date. Upon depletion, the lot is designated in the catalog as “Previous Lot” and a “Valid Use Date” is assigned. USP publishes the Catalog of Reference Standards bimonthly. The most current version of the catalog can be found on the USP Web site at www.usp.org. The user is responsible for ascertaining before use that the USP RS lot of interest currently carries official status, either as a “Current Lot” or as a “Previous Lot” within the Valid Use Date.

Many compendial tests and assays are based on comparison of a test specimen with a USP RS. In such cases, measurements are made on preparations of both the test specimen and the Reference Standard. Where it is directed that a Standard solution or a Standard preparation be prepared for a quantitative determination by stepwise dilution or otherwise, it is intended that the Reference Standard substance be accurately weighed (see Weights and Balances 41 and Volumetric Apparatus 31). Due account should also be taken of the potential errors associated with weighing small masses (see also Section Adjustments to Solutions in the General Notices and Requirements). Reference Standards that are defined on a content-per-container basis are an exception, as noted above.
USP RS instructions for use include the following:
  • As Is: Use without any prior treatment or correction for volatiles. This is the preferred option, and is selected whenever valid data indicate that the volatiles content is constant over time.
  • Dry Before Use: Use immediately after drying under stated conditions. Drying should not be performed in the original container. A portion of the material should be transferred to a separate drying vessel.
  • Determine Water Content Titrimetrically At Time of Use: Use with a correction for the water content or the loss on drying, determined on a separate portion of material. Where the titrimetric determination of water is required at the time a Reference Standard is to be used, proceed as directed for Method I under Water Determination 921. Instrumental or microanalytical methods are acceptable for this purpose. When using typical amounts (about 50 mg of the Reference Standard), titrate with a 2- to 5-fold dilution of the reagent. Where the determination of the loss on drying on a separate portion of USP RS is required, proceed as directed on the label. Sample sizes smaller than those required in the general test chapter Loss on Drying 731 may be used for a USP RS provided that the user can obtain a sufficiently accurate result.
Whenever the labeled directions for use require drying or a correction for volatiles, it should be performed at the time of use. Further experimental details should be controlled by the user's Standard Operating Procedures and good laboratory practices.

USP RS should be stored in the packaging configuration provided by USP (e.g., vials that are packaged in hermetically sealed bags). When special storage conditions are specified, label directions should be followed. Unopened vials should be stored as indicated on the label. The user is responsible for ensuring that the contents of opened vials continue to be suitable for their intended use and that value assignment and uncertainty information are maintained.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
General Chapter Barbara Jones, Ph.D.
Vice President, Reference Standards Evaluation
(RS2010) Reference Standards 2010
USP35–NF30 Page 41
Pharmacopeial Forum: Volume No. 35(6) Page 1507