Despite all the advances in laboratory medicine, microscopic review of the peripheral blood smear is still highly informative and clinically relevant and remains an indispensable diagnostic tool. In a pediatric hospital, a common and challenging request is the evaluation of a newborn blood smear. This chapter provides an overview of some of the characteristics of a newborn smear and common reactive conditions.

Blood smear evaluation is often performed for three essential reasons: first, to clarify a flagged result such as immature cells or a low platelet count to rule out pseudo-thrombocytopenia from platelet clumping; second, to evaluate the morphology of red blood cells, white blood cells, and platelets; and, third, to confirm morphologic findings identified by lab staff or an instrument [1]. The latter may be requested by a physician due to a clinical suspicion or by members of the laboratory staff for review of an abnormal finding.