The nine procedures previously described are the basic tests of hematology. Careful selection and interpretation of these procedures can go far toward solving the vast majority of hematologic problems. Other tests may be ordered to confirm or exclude a diagnosis suggested by the results of preliminary studies. These other tests will be discussed in association with the diseases in which they are useful.

However, once again certain points should be made. Laboratory tests in hematology are no different from any other laboratory tests. Two or more tests that yield essentially the same information in any particular situation should not be ordered. For example, it is rarely necessary to order Hb and Hct determinations and RBC count all together unless indices are needed. In fact, either the Hb or the Hct alone is usually sufficient, although initially the two are often ordered together as a check on each other (because it is the least accurate, the RBC count is rarely helpful). Both the WBC count and differential are usually done initially. If both are normal, there usually is no need to repeat the differential count if the total WBC count remains normal and there are no morphologic abnormalities of the RBCs and WBCs.

Another point to be stressed is the proper collection of specimens. The timing of collection may be extremely important. Transfusion therapy may cause a megaloblastic bone marrow to lose its diagnostic megaloblastic features, sometimes in as little as 12 hours. On the other hand, transfusion will not affect a bone marrow that has no iron. Capillary blood (finger puncture) is best for making peripheral blood smears because oxalate anticoagulant causes marked artifacts in WBC morphology and even will slightly alter the RBC. Ethylenediamine tetraacetic acid (EDTA) anticoagulant will cause a false decrease in Hct values (Hb is not affected) if the amount of blood collected is less than one half the proper volume (for the amount of EDTA in the tube). When capillary (finger puncture) blood is used to obtain the Hct or Hb values or the cell counts, too much squeezing of the finger or other poor technique may result in falsely low values caused by dilution of the blood by tissue juice. On the other hand, dehydration may result in hemoconcentration and produce falsely high values. This may mask an anemia actually present; or another determination after the patient is properly hydrated may give the false impression of a sudden drop in values, as might otherwise come from an acute bleeding episode. Very severe hemoconcentration may simulate polycythemia.