Introduction

It is often easiest to evaluate a bone marrow specimen by comparing it to what would be expected in the normal bone marrow (Brown & Gatter, 1993; Bain, 1996). The initial evaluation on low magnification includes the assessment of sample adequacy and marrow cellularity. The latter is usually based on the biopsy. Estimates of cellularity on aspirate material have been described (Fong, 1979) but may be unreliable in variably cellular marrows (Gruppo et al., 1997). The normal cellularity varies with age (Table 2.1), and evaluation of cellularity must always be made in the context of the patient's age (Hartsock et al., 1965) (Fig. 2.1). The marrow is approximately 100% cellular during the first three months of life, 80% cellular in children through age 10 years; it then slowly declines in cellularity until age 30 years, when it remains about 50% cellular. The usually accepted range of cellularity in normal adults is 40–70% (Hartsock et al., 1965; Gulati et al., 1988; Bain, 1996; Friebert et al., 1998; Naeim, 1998). The marrow cellularity declines again in elderly patients to about 30% at 70 years. Because of the variation in cellularity by age, the report should clearly indicate whether the stated cellularity in a given specimen is normocellular, hypocellular, or hypercellular.

Estimates of cellularity may be inappropriately lowered by several factors. Subcortical bone marrow is normally hypocellular, and the first three subcortical trabecular spaces are usually ignored in the cellularity estimate (Fig. 2.2).