The major emphasis in hematology is placed on the three cellular elements of the blood—red blood cells (RBCs), white blood cells (WBCs), and platelets. Each of these elements is discussed subsequently in separate chapters, beginning with disorders of the RBCs. The noncellular elements of the blood (fluid, electrolytes, plasma proteins, and other constituents) are included in later chapters.

Several tests form the backbone of laboratory diagnosis in hematology.

Hemoglobin (Hb) is the oxygen-carrying compound contained in RBCs. The amount of hemoglobin per 100 ml of blood can be used as an index of the oxygen-carrying capacity of the blood. Total blood Hb depends primarily on the number of RBCs (the Hb carriers) but also, to a much lesser extent, on the amount of Hb in each RBC. Depending on the method used and the care with which the laboratory checks its instruments, manual Hb methods using a spectrophotometer are accurate to 4%-5%, and automated cell counters are accurate to about 2%-3%.

Reference values are most frequently quoted as 14-18 gm/100 ml (140-180 g/L) for men and 12-16 gm/100 ml (120-160g/L) for women (100 ml = 1 dl; 1 gm/100 ml = 1 gm/dl = 1gm%). Some reports indicate lower values, especially in women, so it might be better not to consider a patient anemic until the Hb level is less than 13 gm/100 ml in men and 11 gm/100 ml in women. Infants have different reference limits. Both boys and girls have about the same Hb levels until approximately age 11, after which male values slowly become higher. In one study, adult levels were attained at age 15 in women and at age 18 in men. African American values average 0.5-1.0 gm (5-10 g/L) less than values for Europeans at most ages. The reason for this difference, and how much of it (if any) can be explained by a higher incidence of chronic iron deficiency, is still undecided. In addition, several investigators have found a significant decrease in Hb levels (as much as 1.0 gm) between a sample obtained after some time in the upright position and another obtained later after overnight bed rest.

Several studies have shown that a diurnal variation in Hb and hematocrit (Hct) exist, with the peak at about 9 A.M. and the nadir about 8 P.M. The average Hb difference is about 1.0 gm/100 ml (SI 10 g/L; literature average difference, 0.34-1.5 gm/100 ml; SI 3.4-15 g/L). The diurnal change and the amount of change is not always constant or even constantly present in the same person every day; and the regularity with which it occurs varies considerably from person to person.

There is some evidence that heavy smokers have increased Hb concentration compared with nonsmokers; reported increases range from 0.5-2.2 gm/100 ml (5-20.2 g/L). In pregnancy, the Hb concentration slowly decreases because of dilution from increasing plasma volume, with values as low as 10.0 gm/ 100 ml (100 g/L) being considered normal in the third trimester. There is controversy whether Hb and Hct reference values should be lower in the elderly. At present the majority of investigators would not use a different reference range. High WBC counts may falsely increase Hb in many automated and some manual methods by creating turbidity in the measurement solution.

In newborns, capillary (heelstick) Hb and Hct values are higher than venous blood values. The average neonatal difference between capillary and venous Hb levels is 3.5 gm/dl (30.5 g/L) but varies from 1-10 gm/dl, with a corresponding difference in the Hct values. The increase in capillary Hb concentration tends to be greater in smaller and sicker infants. The difference between heelstick and venous Hb levels becomes less each day and virtually disappears by the fifth day of life. Neonatal Hb concentration depends to some extent on the amount of blood received from the umbilical cord before the cord is clamped.

Changes in Hb Not Due to Blood Loss or Polycythemia
INCREASED
High WBC count
Heavy smoking
Dehydration
DECREASED
Children
Recumbent from upright position
Pregnancy
Diurnal variation (evening)
African Americans
Female sex
Intravenous fluids

Several studies have reported that Hb is more sensitive and accurate than Hct in detecting anemia in adults and newborns.

Fetal RBCs have a life of 60-70 days compared to 90-120 days in the adult. Prematurely born infants may have RBC lifespans of only 35-50 days. At birth, 60%-80% of infant Hb is Hb F rather than Hb A, and the mean full-term Hb value is about 19 gm/100 ml. Hemoglobin slowly falls to levels of about 10-11 gm/100 ml by age 2-3 months (about 8 gm/100 ml in premature infants), begins to slowly increase between age 1 and 5 years, and reaches adult values during puberty.