Diagnosis from the blood smear.

A related slide show is available at www.nejm.org n examination of the blood smear (or film) may be requested by physicians or initiated by laboratory staff. With the development of sophisticated automated blood-cell analyzers, the proportion of blood-count samples that require a blood smear has steadily diminished and in many clinical settings is now 10 to 15 percent or less. Nevertheless, the blood smear remains a crucial diagnostic aid. The proportion of requests for a complete blood count that generate a blood smear is determined by local policies and sometimes by financial and regulatory as well as medical considerations. For maximal information to be derived from a blood smear, the examination should be performed by an experienced and skilled person, either a laboratory scientist or a medically qualified hematologist or pathologist. In Europe, only laboratory-trained staff members generally “read” a blood smear, whereas in the United States, physicians have often done this. Increasingly, regulatory controls limit the role of physicians who are not laboratory-certified. Nevertheless, it is important for physicians to know what pathologists or laboratory hematologists are looking for and should be looking for in a smear. In comparison with the procedure for an automated blood count, the examination of a blood smear is a labor-intensive and therefore relatively expensive investigation and must be used judiciously. A physician-initiated request for a blood smear is usually a response to perceived clinical features or to an abnormality shown in a previous complete blood count. A laboratory-initiated request for a blood smear is usually the result of an abnormality in the complete blood count or a response to “flags” produced by an automated instrument. Less often, it is a response to clinical details given with the request for a complete blood count when the physician has not specifically requested examination of a smear. For example, a laboratory might have a policy of always examining a blood smear if the clinical details indicate lymphadenopathy or splenomegaly. The International Society for Laboratory Hematology has published consensus criteria (available at www.islh.org) for the laboratory-initiated review of blood smears on the basis of the results of the automated blood count. The indications for smear review differ according to the age and sex of the patient, whether the request is an initial or a subsequent one, and whether there has been a clinically significant change from a previous validated result (referred to as a failed delta check). All laboratories should have a protocol for the examination of a laboratory-initiated blood smear, which can reasonably be based on the criteria of the International Society for Laboratory Hematology. Regulatory groups should permit the examination of a blood smear when such protocols indicate that it is necessary.