Turbidimetry measures the presence of solid particles in an otherwise homogeneous solution (the turbidity). The general procedure involves measuring the wavelength of light passing through the solution using spectrophotometric methods. This reveals the concentration of the substance and the substances present in the solution that are causing the turbidity.
In turbidimetric LAL tests, the LAL reagent is added to the test sample to make a solution. If endotoxins are present in the sample, then the clotting reaction of the LAL reagent produces a solid mass (i.e., the clot or gel) in the solution. The resulting degree of turbidity is subsequently measured to determine the presence and amount of endotoxin.
This analysis can be conducted using either the endpoint or kinetic method. The turbidimetric endpoint method involves taking the turbidity reading at a determined time point. However, this can result in manipulation errors and time-consuming repeat tests if the reading is not precise or taken at the right time. As such, the turbidimetric endpoint method is rarely used, with the kinetic method being the preferred turbidimetric approach and our primary focus.