In Reply.—
No single laboratory test result should ever be taken in isolation, no matter how good or common a test it is. Dr Cohen's letter aptly illustrates some of the limitations of the electronic determination of the MCV. Clearly, many other causes of macrocytosis, real or artifactual, exist besides megaloblastic anemia,1-3 and, in fact, agglutination is one of the rarer ones. Yet, it is also worth remembering that mild macrocytosis can be missed on examination of blood smears, even by well-trained observers. The MCV determination may sometimes be a more sensitive tool, therefore, than even the trained eye.In any event, the important point is that the Coulter counter now routinely provides us with several useful pieces of information, none of which should be ignored. The physician's role is to respond properly to all the signals. Unquestionably, one of the first responses should be to look carefully at