Laboratory evaluation of neutropenia and neutrophil dysfunction

INTRODUCTION

The polymorphonuclear neutrophil (PMN) is the primary defense against bacterial and fungal invasion. Although there are some subtle findings which may lead one to suspect either neutropenia or neutrophil dysfunction as a cause of recurrent infection, there is a great deal of overlap in the clinical presentations with those of other immune deficiency states.

The recognition of neutropenia is straightforward (ie, an absolute neutrophil count <1,500/µL), while the recognition of neutrophil functional deficiency is more complex. Unless there is strong historical evidence to suggest a primary neutrophil dysfunction syndrome, the humoral and T-cell systems should be thoroughly investigated before extensive evaluation of neutrophil function is initiated. (See "Laboratory evaluation of the immune system".)

Throughout the following discussion of the laboratory evaluation of neutrophil function, we have focused on those procedures which are easily carried out in a clinical laboratory. Although a large number of neutrophil function assays have been described, the low frequency of requests makes them inappropriate for most routine clinical laboratories. Only the general aspects of each test will be described here. Information concerning specimen requirements, reagents, and the specific procedures to be followed are available elsewhere [1]. Clinical approaches to the patient with neutropenia or neutrophil dysfunction are discussed separately. (See "Overview of neutropenia" and "Primary disorders of phagocytic function: An overview".)

LABORATORY APPROACH TO THE NEUTROPENIC PATIENT

Neutropenia is defined as an absolute neutrophilic granulocyte count (ANC) of less than 1,500/µL, although there are age and racial differences. (See "Overview of neutropenia".) For example, newborn infants have elevated granulocyte counts in the first days of life, and certain populations of blacks and Yemenite Jews normally have lower granulocyte counts.

The ANC is equal to the product of the white blood cell count (WBC) and the fraction of polymorphonuclear cells (PMNs) and band forms noted on the differential analysis; neutrophilic metamyelocytes and younger forms are not included in this calculation: