Bioburden test: what a microbial count measures, and why it is not a sterility test

What a bioburden test measures

A bioburden test counts the live micro-organisms in or on a sample. The EMA’s sterilisation guideline defines bioburden as the total number of micro-organisms on an item before any sterilisation or bioburden-reduction step. The result is a number of colony-forming units (CFU) per gram, per millilitre or per item. One CFU is one visible colony, grown from one or more cells.

Two sets of methods carry the name:

• Medicines and their ingredients. USP <61>, Microbial Enumeration Tests. ICH lists it as interchangeable with Ph. Eur. 2.6.12 and Japanese Pharmacopoeia 4.05, so the method is the same in all three. A companion chapter, USP <62>, tests for the absence of named organisms.
• Medical devices. ISO 11737-1:2018, amended in 2021, covers counting and characterising viable micro-organisms on a health care product, component, raw material or package. It does not cover viruses, prions or protozoa.

A bioburden test answers “how many?”. A sterility test answers “does anything grow?”.

How a bioburden test works: USP <61>

The harmonised chapter counts mesophilic bacteria and fungi that grow under aerobic conditions. It checks a substance or preparation against a set specification for microbiological quality.

The lab usually dilutes the sample 1 in 10 in a buffer or broth. The default amount is 10 g or 10 mL, with less allowed for very small batches. It then counts by membrane filtration (a filter of 0.45 µm or less, laid on agar), by plate count (at least two plates per medium) or, where nothing else works, by the most probable number method in broth tubes, which the chapter calls generally the least accurate.

Each test gives two counts:

Count	Medium	Incubation	What it includes
TAMC, total aerobic microbial count	Casein soya bean digest agar	30–35 °C for 3–5 days	All colonies on this medium, fungi included
TYMC, total combined yeasts and moulds count	Sabouraud-dextrose agar	20–25 °C for 5–7 days	All colonies on this medium, bacteria included

For plate counts, the lab reads plates with fewer than 250 colonies for TAMC and fewer than 50 for TYMC, and converts the mean to CFU per gram or millilitre.

A count is valid only if the product does not suppress growth. The lab adds no more than 100 CFU of each of five test organisms, from Staphylococcus aureus to the mould Aspergillus brasiliensis, and must recover each within a factor of 2 of a control without product.

USP <62> is a separate test for named organisms, such as Escherichia coli, Salmonella, Pseudomonas aeruginosa, Staphylococcus aureus and Candida albicans. Its result is presence or absence in a stated amount of sample.

How to read a bioburden result

A bioburden number means something only with its unit, its floor and its limit.

• The floor. With the usual 1 in 10 dilution and 1 mL on each plate, one colony stands for 10 CFU per gram. Plates with no colonies give a result of less than 10 CFU per gram, not zero. Filtering a larger amount lowers the floor.
• The amount tested. The default is 10 g. A peptide vial holds milligrams. Less material raises the floor per gram, so the report should state the amount tested and whether results are per gram or per vial.
• The limit. The harmonised text reads limits written as powers of ten this way: a limit of 10¹ CFU allows a count of up to 20, 10² allows up to 200, and 10³ allows up to 2000.

Bioburden vs sterility: the differences

	Bioburden (USP <61>)	Sterility (USP <71>)
Question	How many live aerobic bacteria and fungi are in the sample?	Does anything grow from the tested units?
Meant for	Non-sterile products, ingredients, bulk before a sterilisation step	Products that must be sterile
Media	Casein soya bean digest agar; Sabouraud-dextrose agar	Fluid thioglycollate medium, mainly for anaerobes, at 30–35 °C; soya-bean casein digest medium at 20–25 °C
Time	3–5 days and 5–7 days	At least 14 days
Result	A number in CFU per gram, millilitre or item	Growth or no growth
Pass rule	Count at or under a limit	No growth in any medium

Why a low bioburden count is not a sterility result

• Different organisms and time. The count looks for microbes that grow under aerobic conditions, over 3 to 7 days. The sterility test adds a medium for anaerobes and gives slow growers at least 14 days.
• A floor in place of zero. “Less than 10 CFU per gram” means fewer than the method can see in the amount tested.
• A different decision rule. A count passes at or under its limit, so a passing count can include live microbes: a limit of 10² CFU per gram allows up to 200. A sterility test fails on any growth that is not traced to a fault in the test itself.

Neither test proves a batch is sterile

The EMA guideline is direct: sterility cannot be assured by testing. It comes from a validated, controlled manufacturing process. The guideline defines sterility through a sterility assurance level of 10⁻⁶ or less, which means no more than one non-sterile item in a million.

The sterility chapter agrees: a pass shows only that no organism was found in the units examined. If 1 vial in 100 in a large batch carried live microbes, a test of 20 vials would miss all of them about 82% of the time (0.99 multiplied by itself 20 times). Our guide to endotoxin vs sterility sets out how many units the method tests, and sterility testing for peptides compares USP <71> with PCR and other screens.

Where bioburden testing fits

Bioburden controls materials and steps before something is made sterile, and products never meant to be sterile.

• Before sterilisation. The EMA guideline says bioburden criteria should be set before every sterilisation process. For a solution sterilised by filtration, it accepts in most situations no more than 10 CFU per 100 mL (TAMC), measured on a 100 mL sample of the bulk immediately before filtration.
• Bulk substance for injectables. In its BPC-157 briefing for the July 2026 meeting of its Pharmacy Compounding Advisory Committee, the FDA called bioburden (a microbial enumeration test) and the endotoxin test critical for a bulk substance meant for compounded injections. No certificate of analysis came with the nomination for BPC-157 free base, so the FDA wrote that it was unclear whether either test was in place.
• Non-sterile products. Tablets, capsules and creams are checked against limits for TAMC, TYMC and, where set, the absence of specified organisms.

For a peptide brand, a bioburden count on bulk powder shows the microbial load of the raw material before vials are filled; see bulk peptide powder testing. For a finished vial sold as sterile, the matching test is Sterility (USP <71>, 14 days). Endotoxin, which a count does not measure, needs its own LAL test. A lab result shows what was in the sample tested. It cannot say whether a product is safe to use.

How peptide labs name microbial tests

Peptide labs list microbial counts under several names. In our September 2026 review of public certificates from 25 labs and one rating site, two labs printed TAMC and TYMC counts under the name sterility. The names below are from the labs’ own sites in September 2026:

Lab	Name on the lab’s site	What it is	USP <71> also listed
Vanguard Laboratory	Bioburden (USP 61), in its Gold package	Microbial count	Yes, a 14-day option with one extra vial
TrustPointe Analytics	USP <61> Nonsterile Microbial Enumeration	TAMC and TYMC; lead time 10–14 days	Yes; lead time 14–21 days
Liquilabs	Bioburden	TAMC and TYMC plate count; its reports cite USP <61>	Not listed
Testides Analytical	Microbial enumeration and specified organisms	USP <61> and <62>	Not listed
Janoshik	Sterility testing (TAMC+TYMC)	Culture count, pass or fail; 14 days; two unopened vials	Not listed
PharmLabs	Sterility (TAMC & TYMC)	Microbial count; 3–5 business days; 1.0 g of sample	Not listed

Several labs make the line easy to see. Vanguard states on its site that bioburden is not sterility. TrustPointe’s product text contrasts its USP <71> pass or fail result with the TAMC and TYMC result of USP <61>. Janoshik and PharmLabs put TAMC and TYMC in the test name, so the method is visible to anyone who reads it.

Before you order a microbial test, ask which chapter it follows (USP <61>, <62> or <71>), how many days it incubates, how much sample it uses, and whether the report gives a CFU number with its floor or only a pass.

See how to read a COA for the rest of a report, and which labs run a real 14-day sterility test for USP <71> offers.

What HK Analytical offers

We do not offer bioburden counts or microbial screens. Our test for live microbes is Sterility (USP <71>, 14 days): the tested units incubate for at least 14 days, and the result covers only those units. It needs its own unopened vials. Endotoxin (LAL) reports EU per mg and EU per vial. Both can go in one order with the rest of our tests, in any mix per sample. Results within days of receipt. Sterility (USP <71>, 14 days) is the one test that takes 14 days, because the method incubates for 14 days. Every number comes with its raw data, in a signed record you can check on /verify. For a count on bulk powder, ask a lab that lists USP <61> by name.