The polymorphonuclear neutrophil (PMN) is the primary defense against bacterial and fungal invasion. Although there are some subtle findings that 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 <1500/microL), 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".)
Although a large number of neutrophil function assays have been described, the low frequency of requests makes them inappropriate for most routine clinical laboratories, and many are used mostly for research purposes. 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 . Clinical approaches to the patient with neutropenia or neutrophil dysfunction are discussed separately. (See "Overview of neutropenia in children and adolescents" 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 1500/microL, although there are age and racial differences. (See "Overview of neutropenia in children and adolescents", section on 'Definitions' and "Approach to the adult with unexplained neutropenia", section on 'Terminology and definition 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: