The McFarland Equivalence Standards are intended to be part of a quality control program for adjusting densities of bacterial suspensions that are used for identification and susceptibility testing. Each standard is made from different concentrations of latex beads mixed in a buffer liquid. The original McFarland Standards were made from the combination of Barium chloride and Sulfuric acid that result in a flocculate. Problems were encountered with this technique which included instability, storage, and reproducibility of the resulting suspension. These problems have been overcome by using latex particles in a buffer solution to make Colorimeter and McFarland Standards.
The McFarland Standard tubes contain latex particles suspended in a special buffer that are adjusted to an acceptable transmission range using a spectrophotometer at a wave length of either 600 or 625 nm. A bacterial suspension once adjusted to the same turbidity of a McFarland Standard produces expected bacterial plate counts and can be used in a variety of identification or susceptibility kits and methods.