Enzyme immunoassay of lactoferrin in newborn term infants: reference values and influence of diet.

Plasma lactoferrin is an acute phase protein which may have a variety of roles in the anti-infective process. As the diagnosis of infection in infants is particularly difficult, measurement of this protein could make a useful contribution, however previous reference values for infants were unsatisfactory. A solid phase enzyme immunoassay for plasma lactoferrin which required only a small specimen of blood was established. It was linear up to 800 micrograms/L, with a recovery of between 96 and 99%, and had a between batch precision of between 10 and 15%. Using this method we have determined the level of circulating lactoferrin in term infants during the first three weeks of life to be (mean +/- SD) 385 +/- 113 micrograms/L which is significantly (P less than 0.005) higher than that found in adults (venous plasma 122 +/- 40 micrograms/L; capillary plasma 107 +/- 73 micrograms/L). Plasma lactoferrin levels were still significantly higher than those in adults at weeks 7 and 11 (267 +/- 176 micrograms/L and 269 +/- 163 micrograms/L respectively) but not at week 15 (176 +/- 165 micrograms/L). There were no differences between infants fed breastmilk and those fed on infant formulas. These findings are discussed in terms of the possible origin and role of lactoferrin in the newborn.