Direct spectrophotometry of total bilirubin is measured at which wavelength?

• A. 450 nm
• B. 560 nm
• C. 600 nm
• D. 405-460 nm

Answer: (A)

Direct spectrophotometric measurement of bilirubin relies on the color produced when bilirubin reacts with a diazo reagent to form azobilirubin. That colored product has its strongest (peak) absorption in the blue-green region, around 450 nm. Reading at about 450 nm gives the clearest, most sensitive, and most linear signal for bilirubin concentration in clinical samples. While other wavelengths near this region (the 405–460 nm range) cover the same area, the single best choice is 450 nm because it aligns with the peak absorbance of the azobilirubin color.