What is the Difference Between Limit Test and Assay

The main difference between limit test and assay is that limit test allows the identification and control of small quantities of impurities present in a substance whereas an assay mainly allows the identification and determination of the key component of a sample. Furthermore, limiting tests are responsible for the quantification of impurities in a sample compared to a standard. In contrast, an assay both qualify and quantify the key component of the sample.

Limit test and assay are two methods involved in the determination of components in a sample. They are important analytical procedures in drug development.

Key Areas Covered

1. What is a Limit Test
     – Definition, Features, Importance
2. What is an Assay
     – Definition, Features, Importance
3. What are the Similarities Between Limit Test and Assay
     – Outline of Common Features
4. What is the Difference Between Limit Test and Assay
     – Comparison of Key Differences

Key Terms

Assay, Impurities, Limit Test, Qualitative, Quantitative, Target

Difference Between Limit Test and Assay - Comparison Summary

What is a Limit Test

A limiting test is a test which investigates the quantity of foreign matters or in other words, impurities found in a particular compound. Therefore, this type of test is mainly quantitative. Generally, impurities are present in small quantities as they are not the main functional component of the compound. More often, limiting tests are performed to quantify the inorganic impurities such as chloride, sulfate, iron, arsenic, and other heavy metals. Also, these tests allow controlling the impurities by allowing the comparison of the amount of impurities with standards. Generally, the importance of a limit test is to identify the harmful amount of impurities and both the avoidable and the unavoidable amount of impurities.

What is an Assay

An assay is an analytic procedure used to characterize the main functional component of a sample. Therefore, it is can be a quantitative and/or qualitative test. Though the original application is to determine the metal purity, assays are widely used in other areas as well, which include laboratory medicine, pharmacology, environmental biology, immunology, molecular biology, and biochemistry.

What is the Difference Between Limit Test and Assay

Figure 1: Results of an Immunoassay

The measuring entity of an assay is known as the analyte or the target of the assay. It can either a drug, biochemical substance or even a cell of a living organism. Generally, in an assay, all exogenous reactants or reagents, as well as the experimental conditions, are kept constant while only the target of the assay varies either in quantity or quality. Some examples of assays used to analyze proteins as the target are Lowry protein assay, Bradford assay, BCA assay, etc.

Similarities Between Limit Test and Assay

  • Limit test and assay are two analytical procedures involved in the identification of components of a sample.
  • They play a key role in drug development procedures.
  • Also, both procedures can be either quantitative or semi-quantitative.

Difference Between Limit Test and Assay


A limit test refers to a quantitative or semi-quantitative test designed to identify and control small quantities of impurity which is likely to be present in a substance while an assay refers to an investigative (analytic) procedure for qualitatively assessing or quantitatively measuring the presence, amount, or functional activity of a target entity (the analyte). Thus, this is the main difference between limit test and assay.

Target Component

Target component is also a major difference between limiting test and assay. Generally, limit test targets an impurity present in a substance in a small quantity while an assay targets the main functional component of the sample.

Examples of Target Component

Impurities like heavy metals, iron, sulfate, chloride, etc. are the targets for limit test while DNA, RNA, protein, carbohydrates, antibodies, etc. are the targets for assays.

Type of Test

Moreover, limit tests are mainly quantitative or semi-quantitative while assays are either quantitative, semi-quantitative or qualitative. 


Limit tests are important to standardize the impurities while assays are important to characterize the main functional component of a sample. Hence, this is another difference between limit test and assay.


A limit test is a type of quantitative testing procedure used to identify the quantity of an impurity in a substance. In comparison, an assay is both quantitative and qualitative testing procedure responsible for the characterization of the main functional component of a sample. Both limiting tests and assays are important in various analytical procedures including drug development. However, the main difference between limit test and assay is the target compound and the type of test.


1. “Recently Added Formulas in Chemistry.” Web Formulas, Available Here.
2. Dutta, Siddhartha. “Assays, Types of Assays, Principle and Prerequisites of Assays and Bi…” LinkedIn SlideShare, 5 Aug. 2017, Available Here.

Image Courtesy:

1. “ELISA” By BiotechMichael – Own work (Public Domain) via Commons Wikimedia  

About the Author: Lakna

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things

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