When you manufacture or distribute dietary supplements, proving the microbiological quality of every batch isn’t optional—it’s a fundamental safety requirement. Two USP chapters often come up in conversations with quality teams: the harmonized <61> Microbial Enumeration Tests [1][2] and the supplement-focused <2021> Microbial Enumeration Tests—Nutritional and Dietary Supplements [4]. The practical question is which one applies to your product and whether your current testing protocol needs to change.

Both chapters are used to count viable aerobic microorganisms in nonsterile products, but they sit in different parts of the USP–NF and carry important distinctions in scope. Let’s break down what each chapter requires and how they differ.

What Is USP <61> and What Does It Measure?

USP <61> is the harmonized general chapter for microbial enumeration of nonsterile products, adopted through the Pharmacopeial Discussion Group and aligned with the European and Japanese pharmacopoeias [2]. The chapter describes quantitative methods to enumerate mesophilic bacteria and fungi that may grow under aerobic conditions [1]. It is designed to assess whether a product meets an established specification for microbiological quality [1].

The methods described in <61> include membrane filtration, pour‑plate and surface‑spread plate‑count techniques, and the most‑probable‑number (MPN) method [1]. Testing produces two primary results:

Total aerobic microbial count (TAMC), determined on Soybean–Casein Digest Agar after incubation at 30–35 °C for 3–5 days [1].
Total combined yeasts and molds count (TYMC), determined on Sabouraud Dextrose Agar after incubation at 20–25 °C for 5–7 days [1].

When you read a certificate of analysis, TAMC and TYMC are the numbers plotted against the acceptance criteria. Colonies of fungi that appear on the bacterial medium are counted as part of TAMC, and bacterial colonies that grow on the fungal medium count toward TYMC [1].

What Is USP <2021>?

USP <2021> sits in the dietary supplements general chapters of the USP–NF and provides tests for “the estimation of the number of viable aerobic microorganisms present in nutritional supplements of all kinds, from raw materials to the finished forms” [4].

An important feature of <2021> is that it explicitly permits alternative methods, provided they have been properly validated to give equivalent or better results [4]. This flexibility can be valuable when you work with ingredients that have inherent antimicrobial activity or unusual physical forms.

How the Two Chapters Compare

The primary difference comes down to scope. USP <61> is a general chapter written for any nonsterile product—pharmaceuticals, consumer health items, and more [1]. USP <2021> narrows the focus to nutritional and dietary supplements [4].

While <61> lays out specific enumeration methods such as membrane filtration and plate counts [1], <2021> does not prescribe a single testing technology; it allows validated alternative methods that can be adapted to supplement matrices [4]. For a supplement manufacturer, <2021> is the more directly applicable chapter, because its stated scope is nutritional supplements from raw materials to finished forms [4].

Method Suitability and Negative Controls

Every time you test a product, the counting method must be shown to work in the presence of that specific matrix. USP <61> requires that you verify the method’s ability to detect microorganisms without inhibition by the product [1]. This suitability study typically uses an inoculum of not more than 100 CFU of each test strain, and recovery must fall within a factor‑of‑2 range around the inoculum count—for example, with a 100‑CFU spike, acceptable recovery lies between 50 and 200 CFU [3].

Negative controls are mandatory every time you test a product [3]. The control uses the same diluent and procedure but without the sample, and it must show no growth of microorganisms [1]. A failed negative control means the test is invalid and requires an investigation [1][3]. Growth promotion must also be verified on every new batch of culture medium before routine use [3].

Dietary supplement matrices—herbal powders, protein blends, chelated minerals, oil‑based softgels—often suppress microbial growth. In those situations you may need to neutralize, dilute, or filter the sample to achieve valid recovery. If inhibition cannot be overcome, <61> directs you to use the method and test conditions that come closest to the acceptance criteria [1]. The same principle applies when working under <2021>, and the chapter’s allowance for validated alternative methods [4] offers a practical way forward.

A 2025 Clarification to <61>

A correction to the harmonized standard for <61> was approved with an official date of May 1, 2025 [2]. The change involves a clarification to the reading procedure that is performed when verifying the suitability of the membrane filtration method [2]. This update does not introduce new limits or acceptance criteria; it simply refines how laboratories execute and interpret that portion of the test. Labs that run this chapter for supplement testing should confirm their SOPs reflect the clarified procedure.

Choosing the Right Test for Your Supplement

If your product is a dietary supplement—whether a botanical, vitamin, mineral, amino acid, or combination product—the chapter written for it is USP <2021> [4], so start there when you set your release specifications. If your quality unit already runs the general chapter and the method suitability data are sound, ask your lab whether the supplement chapter is a better fit for your specifications. The most important step is to have a validated, product‑specific microbial enumeration method that consistently demonstrates your batches meet the required limits.

Sources

[1] PDF 61 MICROBIOLOGICAL EXAMINATION OF NONSTERILE … (usp.org, accessed 2026-10-05)
[2] Microbial Enumeration Tests | USP (usp.org, accessed 2026-10-05)
[3] FAQs: Microbial Examination of Nonsterile Products: Microbial Enumeration Tests | USP (usp.org, accessed 2026-10-05)
[4] 〈2021〉 Microbial Enumeration Tests—Nutritional and Dietary Supplements (doi.usp.org, accessed 2026-10-05)

Written by Nour Abochama, Vice President of Operations, Qalitex Laboratories. Learn more about our team

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USP <2021> vs USP <61>: Microbial Enumeration for Supplements