Chapter 11: Laboratory Diagnosis: Clinical Hematology

INTRODUCTION

BLOOD COLLECTION

Venipuncture is discussed in detail in Chapter 19. The CBC sample is usually venous blood drawn with a 22-gauge or larger needle. Smaller-caliber needles can cause hemolysis. For a routine CBC, venous blood must be placed in a purple top tube, containing an anticoagulant (EDTA), which should be gently mixed with the blood. The CBC should be performed within 24 hours of collection. Most samples for coagulation studies are submitted in a blue top (citrate) tube. (See Chapter 19, Figure 19-14 for detailed description of blood collection tubes.) If a capillary or heelstick sample (see Heelstick and Fingerstick (Capillary Blood Sampling)) is used, the hematocrit may be falsely low or high due to preanalytical collection variables (such as milking the finger to get the sample or sludging of the RBCs).

BLOOD SMEARS: WRIGHT STAIN

When a CBC with differential is ordered, the WBC differential is most commonly performed by an automated CBC instrument (automated differential). Based on the instrumentation and laboratory-specific alerts/flags, a manual differential may be performed. The red cell, white cell, and platelet morphologies are accessed by reviewing a Wright-stained blood smear. Such morphologic assessment can assist in the evaluation of red and white blood cell disorders. The slide is usually available for review by students and house staff upon request. The main benefit is to allow identification of abnormal cells and other subtleties that may not be detected with automated systems (Figure 11-1). Characteristics of some abnormal WBCs are shown in Figure 11-2.