Determination of total protein in human milk: comparison of methods.

We evaluated four spectrophotometric methods for suitability in determining total protein in human milk samples. Results were compared with those by an established method for the determination of total nitrogen (micro-Kjeldahl). Values for total protein by all four methods significantly correlated with micro-Kjeldahl. For the biuret assay, Lowry-Peterson assay, Bio-Rad Coomassie Blue assay, and Pierce BCA assay, the correlation coefficients were 0.96, 0.97, 0.89, 0.99, respectively. We also assessed the sensitivity of each assay to several purified human milk proteins. The Pierce BCA assay showed the least difference in values among different types of protein. For this reason, and because it showed the greatest precision (with acceptable constant and proportional error), we recommend the BCA method.