There are several methods to detect isoniazid and rifampicin in blood, one of which is spectrophotometry. Spectrophotometry is a technique that uses light to determine the concentration of a substance in a sample.

To detect isoniazid and rifampicin in blood by spectrophotometry, a blood sample is first collected and the drugs are extracted from the blood into a solvent. The extracted solution is then analyzed using a spectrophotometer. The spectrophotometer measures the amount of light absorbed by the extracted solution at a specific wavelength. The light absorption at this wavelength is proportional to the concentration of the drugs in the sample.

To quantify the concentration of isoniazid and rifampicin in the blood sample, a calibration curve is first prepared by measuring light absorption for a series of known concentrations of the drugs in the solvent. The absorption of light for the unknown sample is then measured and the concentration of the drugs is calculated using the calibration curve.

It is important to note that this detection method requires a laboratory setting with specialized equipment and trained personnel to ensure accurate results.