Assay

An assay is an investigative (analytic) procedure in laboratory medicine, pharmacology, environmental biology, and molecular biology for qualitatively assessing or quantitatively measuring the presence or amount or the functional activity of a target entity (the analyte) which can be a drug or biochemical substance or a cell in an organism or organic sample.Generally in an assay the quantity of the exogenous materials (the reagents) are kept fixed (or in excess) so that the quantity (and quality) of the target is the only limiting factor for the reaction/assay process, and the difference in the assay outcome is used to deduce the unknown quality or quantity of the target in question. Some assays (e.g., biochemical assays) may be similar to or have overlap with chemical analysis and titration. But generally assays involve biological material or phenomena which tend to be intrinsically more complex either in composition or in behavior or both. Thus reading of an assay may be quite noisy and may involve greater difficulties in interpretation than an accurate chemical titration. On the other hand older generation qualitative assays especially bioassays may be much more gross and less quantitative (e.g., counting death or dysfunction of an organism or cells in a population, or some descriptive change in some body part of a group of animals).
An assay (analysis) is never an isolated process and needs to be preceded by certain necessary procedures which are the preanalytic steps and must be followed by certain necessary post analytic steps. The information communication (eg request to perform an assay and further information processing) or specimen handling (eg collection transport and processing) done before and till the point of beginning of an assay are the preanalytic steps. Similarly after the actual assay is done the result may be documented, verified and transmitted/communicated in steps which are called post-analytic steps related to an assay. Like any multistep information handling and transmission systems, variation and errors in the communicated final results of an assay involves corresponding parts in every such step ie not only analytic variations and errors intrinsic to the assay itself but also variations and errors involved in preanalytic and post analytic steps. Since the assay itself (the analytic step) gets a lot of attention, steps that get less attention by the chain of users ie the preanalytic and the post analytic steps are often less stringently regulated and generally more prone to errors- eg preanalytic steps in medical laboratory assays may contribute to 32-75% of all lab errors

Example of Assay of Tetracycline.



Requirement:
Pipette, volumetric flask, beaker, measuring cylinder, and watch glass

Label claim:
25mg/capsule

Official Limits:
95% – 105%
?
?

Procedure:
Dissolve the content of 1 capsule in 50ml of N/10 HCl in 100ml of volumetric flask and make up the volume up to 1000ml with N/10 HCl. This is 1st dilution pipette out 5ml from solution into another volumetric flask and make up the volume up to 100ml with distill water. This is second dilution. Now collect 10ml from solution. Put it In volumetric flask and add 60ml of distill water and 10ml of N/10 NaOH. It should be added at time of measuring the absorbance because it is time dependent reaction. Now make up the volume up to 100ml with distill water. This is final dilution. Measure the absorbance of resultant solution at 380nm taking as ? . determine the constant of capsule by using formula
Percentage purity:

Preparation of N/10 NaOH:
Dissolve 0.4g of NaOH in 00ml of distill water.

Preparation of N/10 HCl:
Dissolve 0.85ml of HCl in 100ml of distill water.

Result:
The percentage purity of given sample of tetracycline capsule is 100.8% by spectrophotometer.