 Since then, Guthrie card samples have been collected routinely from infants in over 20 countries to screen for phenylketonuria and more recently for congenital hypothyroidism, sickle cell disorders and HIV infection.The limitations of sensitivity and specificity when screening such small volumes of blood restricted the use of dried blood spots for many years.However, recent advances such as the production of monoclonal antibodies, expression of synthetic proteins, and the introduction of the polymerase chain reaction have overcome many of these problems.