Clinical and Laboratory Standards Institute document I/LA31-A--Newborn Screening for Preterm, Low Birth Weight, and Sick Newborns; Approved Guideline addresses newborn screening of preterm, low birth weight (LBW), and sick newborns worldwide to detect treatable conditions before physical damage can occur to the infant. In developed countries, advances in the treatment they receive have improved survival rates, making the early detection of conditions detected by newborn screening even more important. The physiological states associated with preterm, LBW, and sick newborns, and the treatments they receive, directly affect the reliability of results for many conditions screened in public health dried blood spot and newborn hearing screening systems. This guideline describes the effects of maternal and infant conditions, as well as treatments given to newborns. It also provides the rationale for the recommended intervals for screening designed to minimize the risk of missing or delaying a diagnosis in an affected newborn. It is intended for use by those involved in any aspect of newborn screening specimen collection or hearing screening testing and follow-up, including health care providers, parents, public health professionals, and others concerned with the health and welfare of newborns.