Ultrasonic frequency analysis for estimating pH in albumin-rich biofluids.

Ultrasound is known as a non-invasive imaging modality capable of propagating through highly scattering media such as tissue, blood, and other biological fluids, yet currently provides little chemical information. We have developed a straightforward and rapid methodology for estimating pH in albumin-rich biofluids based on analysis of ultrasonic frequencies. Albumin is the most abundant protein in serum and undergoes conformational changes with pH. It was shown that when ultrasound propagated through albumin solutions, the attenuation of collected ultrasound signals increased with pH. By measuring the ultrasound frequency spectra at several albumin concentrations and pH values, the pH of the solutions could be determined by multilinear regression. Differences in absolute protein content contributed to signal differences in the frequency profiles and were minimized through normalization of each spectrum by the sum of all its frequency intensities. This strategy was applied to human serum samples from multiple donors, for which a multilinear regression model was developed with a coefficient of determination (R(2)) of 0.93 and a standard error of estimate (SEE) of 0.08 pH units. The use of albumin as a pH indicator opens the doors for estimations in other albumin-rich media, such as amniotic fluid and cerebrospinal fluid.