It is a turbidity standard test for the preparation of bacterial suspension to carry out any microbial testing.
It is mostly used in microbiology as a reference to adjust the turbidity of bacterial suspensions to standardize the number of bacteria in a given range.
An example of these is the antibiotic susceptibility testing through measuring the minimum inhibitory concentration, if McFarland standard will not be performed prior to the test, the suspension might be too heavy or too dilute and it will lead to an invalid result for any given anti-microbial agent.
McFarland standard mixtures are the amounts of barium chloride and sulfuric acid. Mixing the two compounds forms a barium sulfate precipitate which causes turbidity in the solution. It is also used to make spectrophotometric evaluation of bacterial concentration in water, saline or liquid growth medium.
A standard preparation mixture of 0.5 McFarland are by mixing 0.05 mL of 1.175% barium chloride dihydrate (BaCl2•2H2O), with 9.95 mL of 1% sulfuric acid (H2SO4).
A McFarland 0.5 standard must be prepared with excellent restriction prior to the beginning of susceptibility testing. It must be tightly sealed to avoid evaporation and place it in dark area, the standard can be stored for up to 6 months.