Biochemistry is the study of the chemical changes in the bimolecules. A change in the concentration of these molecules from the normal range is an indication of a possible problem with some part of the body. An analysis of the concentration of these molecules (enzymes/proteins/lipids) gives an estimate of the extent of the problem and the specific area of the body it is affecting.

Clinical Chemistry, also known as Chemical Pathology, Biochemistry or Diagnostic Chemistry, is the division of pathology that deals with observation and investigation of biochemical changes in the body. It involves the detection and measurement of the biochemical constituents of body fluids and excretions. The field of clinical chemistry today embraces knowledge of physiology, medicine, statistics, electronics and other specialities necessary to provide analysis and answers to the wide variety of chemical changes in the body.

Routine Clinical Chemistry is based on the detection of chromophores (dyes) or direct measurement of participants in the chemical reaction, i.e. NADH in enzyme reactions or bilirubin in bilirubin oxidation assays. Medical tests are used to establish diagnosis, determine the best treatment modality, and monitor treatment. Laboratory tests can yield results that are suggestive of specific diseases but are not reliable when used as the sole indicator of disease. Until 15-20 years ago, tests were ordered sequentially to make a diagnosis by a process of elimination. Today the physician is under time constraints and hence order batteries of tests.

Biochemistry is the measurement of colours and is the most widely used method for determining the concentration of biochemical compounds. This important laboratory procedure is based on the principle that when white light passes through a coloured solution, some wavelengths are absorbed more than others. Many compounds though not coloured, they can be made to absorb light at given wavelength at visible spectrum. The extent to which a solution absorbs light depends on the intensity of its colour.

Beer's Law: The proportion of the incident light absorbed by the molecules in a solution is directly proportional to the number of absorbing molecules in the light path. The ability of a substance to selectively absorb certain wavelengths of light while transmitting others is determined by its molecular and atomic structure. The concentration of the absorbing substance is directly proportional to the intensity of the colour of the solution. i.e. as the concentration of the coloured substance in the solution increases, absorbance increases and the amount of light passing through decreases.

Lambert's Law: When monochromatic light passes through a coloured solution, the amount of light absorbed increases with the increase in the thickness of the layer of the solution through which the light passes. Thus, the concentration of the solution is constant while the light path varies.

Beer Lambert Law: Combining Beer's law and Lambert's Law establish the basic principles of colorimetry and spectrophotometry.

This joint law shows that under suitable conditions, the amount of light absorbed by a coloured solution, when illuminated with a light of suitable wavelength, is directly proportional to the concentration of the coloured solution and the length of the light path through the solution. Therefore the amount of light decreases exponentially with the increase in the concentration of the solution and with increase in thickness of the layer of solution through which light passes. Light path can be kept constant by using tubes or cuvettes identical in size and optical density.