Question

Im studying for a lab exam and need some help with concepts. First, what is an 'indirect' quantitative assay ? And how is a standard curve used to quantify unknowns?

Answer 1

Biological assays are a set of techniques which help us compare among the strength of alternative but similar biological stimuli based on the response on a subject. Biological assays can be direct or indirect.
When the response cannot be measured directly, the indirect methods are used to estimate a certain dose with respect to the given response with the help of a dose-response relationship.
In an indirect assay, a dose is prefixed (non-stochastic in general). At each prefixed value of the dose, the response is observed for the included subjects. Hence, the dose in an indirect assay is non-stochastic whereas, the response is stochastic.
The tolerance distribution for a particular sample is obtained from the stochastic response. If the response obtained is now quantitatively variable (magnitude of weight, survival time etc.) then we have an indirect assay.
A standard curve is used as a quantitative technique to know the properties of an unknown sample, from the already administered assay of the samples with same composition but different concentration or other properties.
In a standard curve, firstly a plot is formed with respect to a property of the known concentration of a solution (color, absorbance, transmittance etc.). Then this plot is used to calculate the concentration of the solution of the same components with unknown concentrations.
Data for the known concentrations of the solute are used to make the standard curve. Plot the concentration on the X-axis, and the assay measurement on the Y-axis. Perform the same assay on the unknown solution samples. Locate the measurement on the Y-axis that corresponds to the assay measurement, of the unknown substance and is following a line that intersects the standard curve. The corresponding value on the X-axis is the concentration of the solute in the unknown solution.