Color Atlas of Hematology - Practical Microscopic and Clinical ...

Procedures, Assays, and Normal Values17In sum, a healthy distrust for the single data point is the most importantbasis for the interpretation of all data, including those outside the referencerange. For every sample of drawn blood, and every countingmethod, at least two or three values should be available before conclusionscan be drawn, unless the clinical findings reflect the cytological data.In addition to this, every laboratory has its own set of reference data tosome extent.After this account of the problems and wide variations between differentgroups, the data in Table 2 are presented in a simplified form, with valuesrounded up or down for ease of comparison and memorization. Absolutevalues and the new SI units are given where they are clinically relevant.The Blood Smear and Its Interpretation(Differential Blood Count, DBC)A blood smear uses capillary or venous EDTA-blood, preferably no morethan three hours old. The slides must be grease-free, otherwise cell aggregationand stain precipitation may occur. Unless commercially availablegrease-free slides are used, the slides should be soaked for several hours ina solution of equal parts of ethanol and ether and then allowed to dry. Adroplet of the blood sample is placed close to the edge of the slide. Aground cover glass (spreader slide) is placed in front of the droplet ontothe slide at an angle of about 30. The cover slide is then slowly backed intothe blood droplet. Upon contact, the blood droplet spreads along the edgeof the slide (Fig. 4). Without pressure, the cover glass is now lightly movedover the slide. The faster the cover glass is moved, and the steeper angle atwhich it is held, the thinner the smear will be.The quality of the smear technique is crucial for the assessment, becausethe cell density at the end of the smear is often twice that at the beginning.In a well-prepared smear the blood sample will show a “feathered” edgewhere the cover glass left the surface of the slide. The smear must bethoroughly air-dried; for good staining, at least two hours’ drying time isneeded. The quality of the preparation will be increased by 10 minutes’fixation with methanol, and it will then also keep better. After drying,name and date are pencilled in on the slide.