Sterility Testing and Heavy Metals Testing Explained

Sterility testing checks a sample for viable microorganisms, and heavy metals testing measures toxic elements such as arsenic, cadmium, lead and mercury. The pharmacopeial sterility test, USP <71>, incubates the sample in two growth media for 14 days and looks for growth, either after passing the sample through a membrane filter or by adding it directly to the media. Heavy metals are now measured element by element, usually by inductively coupled plasma mass spectrometry (ICP-MS), under the procedures in USP <233>. On a lab report, both describe the tested sample, not the identity or purity of the compound.

Sterility and heavy metals testing at a glance

Source: USP general chapters <71>, <231>, <232> and <233>, cited by name; Parveen et al. [1].

Research materials: Vortex supplies peptides and related compounds as sealed research reagents. Browse the catalogue at /shop/. Published lab reports, including the contaminant tests discussed below, are at /coa/.

What sterility testing measures

A sterility test asks one question: does this sample contain microorganisms that can grow? It does not count or, in its compendial form, identify them. A negative result means nothing grew under the test conditions in the portion tested. Because the test examines a sample rather than a whole batch, contamination in a small fraction of containers can be missed. A “no growth” line on a report is a fact about the tested material.

USP <71>: membrane filtration and direct inoculation

The compendial sterility test in USP <71> is a growth test. It relies on visible turbidity in liquid culture media caused by any contaminant that grows [1]. Two media are used together:

• Fluid thioglycollate medium (FTM) supports anaerobic as well as aerobic bacteria. It is incubated at 30-35 °C.
• Soybean-casein digest medium (tryptic soy broth, TSB) supports fungi and aerobic bacteria. It is incubated at 20-25 °C.

Incubation runs for at least 14 days [1]. The sample reaches the media by one of two routes.

Membrane filtration

The sample is dissolved and passed through a membrane filter with a pore size of 0.45 µm or smaller, which retains microorganisms. The membrane is rinsed to remove anything that might inhibit growth, then placed in the two media. Filtration is preferred when the material can be filtered, because it concentrates organisms and removes antimicrobial substances.

Direct inoculation

A measured portion of the sample is added straight into the media. This suits materials that cannot be filtered, but anything that suppresses growth stays in the tube.

How the two compare

In a 1983 comparison on veterinary biologics, closed membrane filtration detected contamination in 29% more batches than direct inoculation, but only one product type could be filtered, so the authors did not recommend it as a universal replacement [3]. A ten-year analysis found no difference between the two methods in the time contaminants took to show visible growth, and found that cutting incubation to seven days would have missed an unacceptable share of contaminants with either method [2].

Method suitability and growth promotion

USP <71> also requires growth promotion testing of the media and a method suitability test, in which no more than 100 colony-forming units (CFU) of challenge organisms are added to the sample and must still grow. Without it, “nothing was there” cannot be told apart from “the sample stopped it growing”.

Rapid sterility methods, including PCR

Fourteen days is a long wait. A 2001 review of rapid microbiological methods grouped them into methods that estimate total microbial content (ATP bioluminescence, impedance, direct viable counts, flow cytometry) and methods that detect specific organisms (PCR and immunoassays), with detection times falling from about 30 hours to 90 minutes in some applications [4].

Some rapid methods still depend on growth but detect it earlier. In a 2011 evaluation at the FDA’s biologics centre, an ATP bioluminescence system detected test organisms at 0.1 CFU more sensitively and faster than the compendial methods, and was judged acceptable as a 5-day sterility method [1]. Two CO2-monitoring systems were faster for most organisms, but the compendial membrane filtration method was more sensitive than both [1].

PCR-based methods

PCR does not wait for growth at all. It amplifies a stretch of microbial DNA, so it reports the presence of the organisms its primers target. Ragheb and colleagues built uniplex, duplex and multiplex PCR assays for four organisms that pharmacopeias name in microbial limit tests (Salmonella spp., Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus), targeting conserved genome regions and validating specificity, sensitivity and robustness [5].

PCR raises two questions a growth test does not. Coverage: an assay finds only what its primers target. Viability: DNA from dead organisms can still amplify, so some methods add an enrichment culture first. A 2026 method combined an automated CO2-based culture enrichment with a qPCR assay for viable microorganisms, removing contaminating DNA and PCR inhibitors during preparation, and reported detection in 3 days with a limit of detection of 1 CFU [6]. A report line reading “Sterility (PCR)” therefore describes a rapid molecular screen, not the 14-day USP <71> culture test, and a report should say which was run.

What heavy metals testing measures

“Heavy metals” is an old phrase. USP <231> tested them together: metals that form coloured sulfides were precipitated and compared by eye with a lead standard, reported as ppm “as lead”. That test could not say which metal was present. It has been replaced by element-specific methods: USP <232> sets limits for named elemental impurities and USP <233> describes how to measure them, in practice by ICP-OES or ICP-MS [7]. The ICH Q3D guideline sets out the same framework internationally.

ICH Q3D groups elements by toxicity and by how likely they are to occur. Class 1 holds the four elements every assessment considers: arsenic (As), cadmium (Cd), mercury (Hg) and lead (Pb). Other classes include catalyst metals and lower-concern elements such as chromium. Many lab reports still use the heading “heavy metals” for what is now an elemental impurities panel.

How ICP-MS measures elemental impurities

Sample preparation

ICP-MS usually needs the sample as a solution. Water-soluble materials are dissolved and diluted. Others are digested in closed vessels with concentrated acid and microwave heating. Mercury can be lost or stick to surfaces, so laboratories commonly stabilise it with hydrochloric acid or gold. Laser ablation ICP-MS skips the solution step: Pluháček and colleagues validated it for 17 elements in pressed pellets of solid pharmaceutical samples against USP <232>/<233> and ICH Q3D requirements, using in-house matrix-matched standards [7].

The measurement

The solution is nebulised into an argon plasma at roughly 6,000 to 10,000 K, hot enough to atomise and ionise almost every element. A mass spectrometer, usually a quadrupole, counts the ions by mass-to-charge ratio, read against calibration standards with internal standard elements to correct for drift and matrix effects.

Interferences

The main analytical trap is a polyatomic ion with the same nominal mass as the analyte. Arsenic is the classic example. Its only isotope is at mass 75, and argon from the plasma combines with chlorine from the sample to form ⁴⁰Ar³⁵Cl⁺, also at mass 75. Colon and colleagues removed the ArCl⁺ interference with a pressurised collision/reaction cell fed with hydrogen and helium, and found rhodium and yttrium the most suitable internal standards for arsenic in a high-sodium, high-chloride matrix [8]. This is relevant to peptide work, because many synthetic peptides are supplied as chloride or other salts.

USP <232>/<233>: limits and validation

USP <232> limits are written for drug products, and ICH Q3D derives them from toxicology and the way a medicine is used. A research reagent is not a drug product, so those limits do not formally apply. When a research-reagent report cites them, it is borrowing a reference point.

USP <233> is the more useful chapter for reading a report, because it defines a valid measurement. For a quantitative procedure, samples spiked with known amounts of each element must return 70% to 150% of the added amount, replicate spiked samples must agree within a relative standard deviation of 20%, and the method must distinguish each element from the others and from the matrix.

How sterility and heavy metals appear on a COA

Not every certificate of analysis includes these tests. When it does, each line should name the method, the specification and the result. Our explainer on what a COA is covers the document as a whole. The examples below come from three reports published on the Vortex Research COA page.

Source: vortexresearch.net/coa/ and the lab reports linked from it, checked October 21, 2026.

Four details deserve a close reading:

• “No Growth” by PCR. “Growth” is culture language, while PCR detects DNA. Neither report states the PCR targets or whether an enrichment step was used, so the line reads best as “no microbial target detected by the lab’s PCR screen”. It is not a USP <71> 14-day result.
• Whose limits. The MOTS-C report states that its elemental acceptance criteria are internal laboratory screening limits for research-use materials, not an evaluation against any pharmacopoeial monograph or product specification.
• “Not detected” has a floor. It means below the method’s detection limit, which these reports do not print. In units, 1 ppm is one microgram per gram, so a 10 mg sample at exactly 1 ppm of lead holds 10 ng of lead.
• Which elements. “All Below Limits” on the 5-Amino-1MQ summary does not list the elements; the lab’s portal copy is where to check.

The GHK-Cu row shows the main rule: a test not on a report was not run on that sample. For the field-by-field method, see how to read a peptide COA.

Why these tests matter for laboratory reagents

For research materials, contaminant tests are about data quality: a contaminant can create a signal that belongs to it rather than to the compound under study.

• Microorganisms in cell work. Contaminating organisms bring their own DNA and cell-wall components into a culture. Bacterial endotoxin, a separate test, is covered in our endotoxin testing explainer.
• Metals and oxidation. Transition-metal ions catalyse oxidation of susceptible residues, with methionine, cysteine, histidine, tryptophan and tyrosine the most reactive [9].
• Unregulated supply chains. A systematic screen of falsified peptide preparations bought from suspected illegal online sources found wide variation in content, low purity in some samples, lead in one sample and inorganic arsenic in several [10].

None of these tests says anything about identity or purity. Those come from mass spectrometry and HPLC, covered in our explainer on HPLC peptide purity.

Frequently asked questions

What is the USP <71> sterility test?

USP <71> is the pharmacopeial sterility test. The sample is passed through a membrane filter or added directly to two growth media, fluid thioglycollate medium and soybean-casein digest medium, and incubated for at least 14 days. Any visible growth is a positive result.

How is PCR sterility testing different from USP <71>?

PCR detects microbial DNA from the organisms its primers target, so it can report within hours to a few days instead of 14 days. It does not on its own show that organisms were alive, which is why some methods add an enrichment culture first. A report should state which method was used.

What does ICP-MS heavy metals testing measure?

ICP-MS measures named elements individually, typically arsenic, cadmium, lead and mercury, plus others such as chromium. The sample is dissolved or digested, ionised in an argon plasma and counted by mass. Results are given in ppm (µg/g) per element, or as “not detected” below the method’s detection limit.

Do research-reagent reports use pharmacopeial limits?

Not necessarily. USP <232> limits are written for drug products. Some research-reagent reports cite them as a reference, and others, such as the ILS MOTS-C report, state that they use internal laboratory screening limits.

About this article: Written by the Vortex Research Editorial Team. USP chapters and ICH Q3D are cited by name; each PMID was checked against PubMed and its abstract. Report details were read from the lab reports on the Vortex COA page on 21 October 2026.

Research materials: Published lab reports, with verification links to each testing lab, are at /coa/. The catalogue is at /shop/.

References

1. Parveen S, Kaur S, David SA, Kenney JL, McCormick WM, Gupta RK. Evaluation of growth based rapid microbiological methods for sterility testing of vaccines and other biological products. Vaccine. 2011;29(45):8012-8023. PMID 21871516. doi:10.1016/j.vaccine.2011.08.055
2. Bathgate H, Lazzari D, Cameron H, McKay D. The incubation period in sterility testing. J Parenter Sci Technol. 1993;47(5):254-257. PMID 8263662.
3. Christianson GG, Koski TA. A comparison of a disposable membrane filtration system with a direct inoculation system for the sterility testing of veterinary biologics. J Biol Stand. 1983;11(2):83-89. PMID 6863323. doi:10.1016/s0092-1157(83)80030-4
4. Jimenez L. Rapid methods for the microbiological surveillance of pharmaceuticals. PDA J Pharm Sci Technol. 2001;55(5):278-285. PMID 11605251.
5. Ragheb SM, Yassin AS, Amin MA. The application of uniplex, duplex, and multiplex PCR for the absence of specified microorganism testing of pharmaceutical excipients and drug products. PDA J Pharm Sci Technol. 2012;66(4):307-317. PMID 22767880. doi:10.5731/pdajpst.2012.00871
6. Wallace NE, Kidwai AS, Knight M, Yang A, von Wintzingerode F. Combining a respiration-based method with universal qPCR for 3-day rapid sterility testing. PDA J Pharm Sci Technol. 2026 (online ahead of print). PMID 42601221. doi:10.5731/pdajpst.2026-000020.1
7. Pluháček T, Ručka M, Maier V. A direct LA-ICP-MS screening of elemental impurities in pharmaceutical products in compliance with USP and ICH-Q3D. Anal Chim Acta. 2019;1078:1-7. PMID 31358206. doi:10.1016/j.aca.2019.06.004
8. Colon M, Hidalgo M, Iglesias M. Arsenic determination by ICP-QMS with octopole collision/reaction cell. Overcome of matrix effects under vented and pressurized cell conditions. Talanta. 2011;85(4):1941-1947. PMID 21872042. doi:10.1016/j.talanta.2011.07.017
9. Li S, Schöneich C, Borchardt RT. Chemical instability of protein pharmaceuticals: mechanisms of oxidation and strategies for stabilization. Biotechnol Bioeng. 1995;48(5):490-500. PMID 18623513. doi:10.1002/bit.260480511
10. Janvier S, Cheyns K, Canfyn M, et al. Impurity profiling of the most frequently encountered falsified polypeptide drugs on the Belgian market. Talanta. 2018;188:795-807. PMID 30029448. doi:10.1016/j.talanta.2018.06.023

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