Types of Assays in Chemistry

Study Notes

Investigative procedure used to measure or assess the presence, amount, or activity of a target entity called the analyte which could be a drug, biochemical substance, or organic sample.

Chemical Assay: Study of separation, identification, and quantification of chemical components in natural and artificial materials.
Immunoassay: Uses the interaction between an antigen and its antibody to identify and quantify the specific antigen or antibody in a sample.
Biological Assay: Testing procedure to estimate the concentration of a pharmaceutical drug in a formulated product or bulk material. The potency of the drug is compared with the standard by observing its effects on animals or human volunteers.

When the chemical method is unavailable, too complex, or insensitive to low doses (e.g., histamine).
When the active principle of a drug is unknown (e.g., insulin).
When the chemical composition of a drug is unknown (e.g., long-acting thyroid stimulator).
When the chemical composition of a drug is variable but it has the same pharmacological action (e.g., cardiac glycosides).
When the active principle cannot be isolated (e.g., posterior pituitary extract, insulin).
When the biological activity of a drug cannot be defined by a chemical assay (e.g., cis and trans forms of methylphenidate).
When it's not possible to separate interfering substances (e.g., Vitamin D).

All bioassays should be compared against a standard drug.
The standard and new drug should be, as far as possible, identical to each other.
The activity measured should be the activity of interest.
The degree of pharmacological response produced should be reproducible under identical conditions (e.g., adrenaline).
The method of comparison preferably (not essentially) tests the therapeutic property of the drug.
Individual variations must be minimized.

Quantal: All or none response in all individuals (e.g., digitalis-induced cardiac arrest in guinea pigs, hypoglycemic convulsions in mice by insulin). It's used to compare LD50 and ED50 and threshold response values.
Graded: The effect is produced gradually depending on the dose (e.g., contraction of smooth muscle preparation).

Biological assay depends on the size of the dose and potency of the drug.
The effect produced in animals might not be the same as that produced in humans (exceptions exist).

To compare a test sample with a standard substance to determine the quantity of the test required to produce an equivalent biological response.
To measure the pharmacological activity of a new or chemically undefined substance.
To test the response of living animals to the toxicity of chemical contaminants.
To investigate the function of endogenous mediators (e.g., in the development of the inflammatory response).
To determine the concentration and potency of an unknown substance.
To improve and maintain standards of environmental conditions that affect people's wellbeing.
To determine the specificity of a compound (e.g., penicillins' effectiveness against G+ve bacteria but not G-ve bacteria).