Harr Clinical Chemistry Practice Test 2025 – All-in-One Study Guide for Exam Success!

The most accurate measurement of inorganic phosphate (Pi) in serum is achieved through the rate of unreduced phosphomolybdate formation at 340 nm. This method relies on the principle of colorimetry, where the formation of a complex between phosphate and molybdate forms phosphomolybdate, which can be detected at a specific wavelength. At 340 nm, the absorbance is measured, providing a direct correlation to the concentration of inorganic phosphate in the serum sample.

This approach is particularly advantageous because it directly relates the absorbance to the concentration of Pi without the need for subsequent reduction steps or additional reagents that could introduce variability. This contributes to the accuracy and reliability of the measurement, making it a preferred method in clinical settings.

The other methods, while used in different contexts, may introduce factors that could affect accuracy. For instance, the measurement of phosphomolybdenum blue at 680 nm, though useful, often depends on a reduction step which can add variability to the results. Similarly, using aminonaptholsulfonic acid to reduce phosphomolybdate is another approach but can complicate the measurement process and potentially lead to less accurate results. Lastly, the formation of a complex with malach