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What is limit of detection in Strep A testing

Videodauer von 3 Min.

03. November 2026

ATEMWEGSGESUNDHEIT

Experten-Perspektive

Limit of Detection: What Clinicians Should Know in Strep A Testing

Limit of Detection: What Clinicians Should Know in Strep A Testing

When evaluating a diagnostic test, clinicians often ask how quickly results will be available, how accurate the test is, and what to do if the result is negative. Another useful question is: What is the test’s limit of detection?

Limit of detection, or LoD, describes the lowest concentration of an organism or target that a test can reliably detect under defined conditions.1 For Group A Streptococcus (Strep A), understanding LoD can help clinicians interpret what a negative result means.

Watch the video: A quick primer on limit of detection and what it can mean in the context of Strep A pharyngitis testing. 

What is the limit of detection?

LoD is defined as “the lowest concentration of analyte that can be consistently detected (typically, in 95% of samples tested under routine clinical laboratory conditions and in a defined type of sample, e.g., plasma, whole blood, urine, cell swabs, or tissue sections).”1

Simply put, it is the smallest amount of a target that a test can reliably detect. In Strep A testing, LoD may be reported as colony-forming units per milliliter, or CFU/mL, which estimates the concentration of viable bacteria in the sample.

  • A test with a lower LoD can detect the target when it is present at a lower concentration.
  • A test with a higher LoD requires more of the target to be present before detection is reliable.
Graph illustrating Low and High Limits of detection

Figure 1. Illustrated example of low vs high LoD

Why does LoD matter for Strep A detection?

Strep A concentration and detectability can vary across throat specimens because of differences in bacterial burden, specimen collection, and test method.2

If the amount of Strep A in a specimen is below a test’s detection threshold, the test may not detect the organism even when some bacteria are present. That is why LoD can be relevant when clinicians think through confidence in a negative result.

 

Is LoD the same as clinical sensitivity?

No. LoD is an analytical sensitivity measure; it asks how little target the test can detect under defined test conditions. Clinical sensitivity asks how often the test is positive among patients who truly have the condition.

Clinical performance also depends on factors such as:

  • How and when the specimen is collected
  • Whether enough material is obtained
  • The patient population being tested
     

For that reason, LoD should be considered together with clinical sensitivity, clinical specificity, workflow, and applicable testing guidance.

 

How should clinicians interpret LoD numbers?

LoD values can be difficult to interpret because the numbers may feel abstract. The key point is directionality: a lower LoD means the test demonstrated detection at a lower organism concentration under the conditions of the study.

A controlled human infection study of Streptococcus pyogenes pharyngitis found that 100,000 to 300,000 CFU/mL caused pharyngitis in 85% of participants, while as little as 10,000 to 30,000 CFU/mL caused pharyngitis in 1 in 5 participants.3 For some on-market rapid antigen tests, published data show LoDs above this threshold (10,000-15,000 CFU/mL)4 which suggests the possibility that the tests may miss detecting Strep A at these levels, reinforcing the value of test sensitivity.

 

What does a lower LoD mean for a negative Strep A result?

A lower LoD may reduce the analytical risk of missing a low-concentration specimen.

 

Example: LoD in molecular Strep A testing

Xpert® Xpress Strep A, a rapid PCR-based test, reports a limit of detection of 9–18 CFU/mL, alongside clinical sensitivity of 99,4% in CLIA-waived testing and 100% in moderately complex testing.5 This low LoD indicates that the test is able to detect very low levels of Strep A, such as early in infection. Taken together, these values can help clinicians understand the analytical sensitivity behind a rapid molecular Strep A result, while considering clinical performance, specimen quality, and the patient’s presentation.

 

Questions to ask when comparing Strep A tests

When reviewing a Strep A testing option, consider:

  1. What is the test’s reported LoD?
    Look at both the number and the unit in which it is reported.

  2. How was the LoD determined?
    Consider the specimen type, bacterial strains, preparation method, and study conditions.

  3. What does the clinical performance evidence show?
    Review clinical sensitivity and specificity in the intended-use population.
     

Also consider what happens after a negative result. Some rapid antigen testing pathways may require culture confirmation in children and adolescents, while some molecular tests such as Xpert® Xpress Strep A do not require culture confirmation for negative results when used according to their instructions for use.

 

Wichtigste erkenntnisse

Limit of detection helps explain how much organism must be present before a test can reliably detect it under defined analytical conditions.

A lower LoD means detection was demonstrated at a lower concentration, but LoD should not be interpreted in isolation. Clinical performance, specimen collection, follow-up requirements, and the patient’s presentation all matter.

For clinicians, LoD is not the whole story, but it is a useful part of understanding what a Strep A test result means, especially when the result is negative.

 

Further reading:

Learn more about Xpert Xpress StrepA 
A PCR test for detecting Streptococcus pyogenes DNA from throat swab specimens5, delivering results in 18-24 minutes.

Expert perspectives:

Die Zeit für die breite Einführung des MDx-Tests für GAS-Pharyngitis ist gekommen: ein Interview mit Scott Sugden, PhD

Revolution bei der Behandlung von Atemwegserkrankungen: Die Kraft der molekularen Schnelltests

When “Rhinovirus” May Not Just Be a Cold: The Diagnostic Blind Spot in RhV/EV Reporting

Clinical impact:

Revolutionizing Rheumatic Heart Disease Prevention: Dr. Asha Bowen’s Vision for Equitable Diagnostics

 

 

About the Author:

Alex Cho trained in general internal medicine and has two decades of experience in primary care, ED and hospital settings, as well as in education, health services research, and healthcare innovation. As one of Cepheid's Medical Science Liaisons, he is passionate about clinician education, with special focus on testing for respiratory illness and in outpatient clinics.

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IVD. In vitro Diagnostic Medical Device. Eventuell nicht in allen Ländern erhältlich.

 

Literatur

1.      Vaks, JE et al. Verification of Claimed Limit of Detection in Molecular Diagnostics, The Journal of Applied Laboratory Medicine, Volume 1, Issue 3, 1 November 2016, Pages 260–270, https://doi.org/10,1373/jalm,2016,020735

2.      Taylor A et al. The utility of rapid group A streptococcus molecular testing compared with throat culture for the diagnosis of group A streptococcal pharyngitis in a high-incidence rheumatic fever population. Journal of Clinical Microbiology. 2021;59(12):e00978-21. doi:10,1128/JCM,00978-21

3.      Osowicki J, Azzopardi KI, Fabri L, et al. A controlled human infection model of Streptococcus pyogenes pharyngitis (CHIVAS-M75): an observational, dose-finding study. The Lancet Microbe. 2021;2(7):e291-e299. doi:10,1016/S2666-5247(20)30240-8 https://www.thelancet.com/journals/lanmic/article/PIIS2666-5247(20)30240-8/fulltext

4.      Chandrasekaran A, Kilic A, Kumar R, Green DA, Wu F, Berry GJ. Evaluation of the Analytical Sensitivity of a Molecular Point-of-Care Assay as Compared to 3 Lateral Flow Assays for Group A Streptococcus. J Appl Lab Med. 2025 Jul 1;10(4):901-910. doi: 10,1093/jalm/jfaf042. PMID: 40248900. https://academic.oup.com/jalm/article/10/4/901/8115967?login=false

5.      Xpert Xpress Strep A Instructions for Use 301-6574, Rev. F; 302-2294, Rev. C.

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