The risk of neutropenia-related infection begins to rise at an ANC near 1000/mm3, rising dramatically below 500/mm3 and more so below 100/ mm3; thus, an ANC of 500/mm3 to 1000/mm3 is considered moderate neutropenia, and below 500/mm3 is considered severe. This corresponds to National Cancer Institute criteria for medication adverse hematologic event reporting: grade 1 toxicity is 1500/mm3 to lower limit of normal, grade 2 is 1000/mm3 to 1500/mm3, grade 3 is 500/mm3 to 1000/mm3, and grade 4 is less than 500/mm3. Beyond the ANC, the risk for infection is greatly influenced by the nature of the underlying problem, the BM myeloid reserve, and whether other risk factors for infection are present (e.g., immunoglobulin deficiency or breaks in mucosal barriers). The course with mild and moderate neutropenia is often benign.
The history focuses on the severity and duration of neutropenia, whether infectious complications have occurred (including stomatitis or gingivitis), and whether prior blood counts can be obtained. A history of drug exposures and their timing is especially relevant, not only for prescribed medications but also for over-the-counter, herbal, and illicit drugs. Symptoms of systemic inflammatory illness, such as arthritis, skin rash, and photosensitivity, can bear on the etiology and significance of the low WBC count. Fevers, weight loss, and sweats could be clues to many disorders, including malignancy. Liver disorders commonly present with cytopenias, so a history of hepatitis, jaundice, and HIV risk factors should be specifically sought. Symptoms of anemia or bleeding could be clues to more general hematologic disorders. As with most hematologic problems, physical examination should devote careful attention to lymph node areas and the spleen. Oropharynx and skin examination also take on added importance.
Peripheral blood smear review is irreplaceable to direct the workup. MDS does not characteristically present with isolated neutropenia, but this occasionally is the predominant presenting feature in this relatively common syndrome affecting older patients. Specific blood smear findings which could suggest MDS include pseudo-Pelger–Huet neutrophils (Fig. 1), hypogranularity, Döhle bodies (Fig. 2), and macrocytic and/or dimorphic RBCs with a hypochromic population. Megaloblastic processes, such as vitamin B12 and folic acid deficiency, similarly do not characteristically present with isolated neutropenia, but this occasionally predominates (especially in the presence of acute infection). Hypersegmentation of neutrophils is always present with megaloblastic processes, including those that are drug-induced (e.g., methotrexate, hydroxyurea). Hypersegmented neutrophils (Fig.3) are also seen with uremia, MDS, and MPN. In a neutropenic patient, one should always specifically look for large granular lymphocytes (LGLs) (Fig. 4). A modest increase could indicate a reactive T-cell or natural killer (NK) cell process, and a more dramatic increase could alert to clonal T-NK or NK cell proliferations (LGL leukemia). Hairy cells (Fig. 5), other morphologic types of circulating lymphoma cells, and blasts are obvious indicators of hematologic malignancy. Morphologic suspicions for such processes could be confirmed by flow cytometry. Reactive lymphocytes may suggest viral infection but also occur with drug reactions and other processes.
Fig1. PELGER-HUET ANOMALY. (Two symmetric round or oval nuclear lobes connected by a thin strand of chromatin.) Hereditary form (involving almost all neutrophils) is benign. Acquired form suggests MDS (as in this hypogranular example), but may be seen transiently with infection, advanced MPNs, myxedema, others. MPNs, Myeloproliferative neoplasms; MDS, myelodysplastic syndrome.
Fig2. DÖHLE BODIES. Most commonly due to infection; myriad other causes include drug-induced (including growth factors), congenital (including MYH9 disorders), MDS. MDS, Myelodysplastic syndrome.
Fig3. HYPERSEGMENTED NEUTROPHIL. Victor Herbert established that hypersegmentation exists if 5% of neutrophils have five lobes or if one has six lobes. This is a sensitive finding for megaloblastic processes (including drug-induced). It is commonly seen with uremia and can be a congenital polymorphism.
Fig4. LARGE GRANULAR LYMPHOCYTES. T-NK lymphocytosis can be reactive or clonal, and are associated with cytopenias, especially neutropenia. NK, Natural killer.
Fig5. HAIRY CELLS. Hairy cell leukemia is associated with cytopenias, including monocytopenia (in the typical form).
Further laboratory testing in patients with neutropenia may often be unnecessary. Especially in mild or moderate cases, the lack of specific diagnostic tests has created some overlap and confusion among what have been called chronic idiopathic neutropenia, chronic benign neutropenia, ethnic neutropenia, and autoimmune neutropenia. Autoimmune neutropenia is a relatively common cause of both mild and more severe neutropenia, but tests for antineutrophil antibodies are not clinically reliable (see later). Serologic tests for antinuclear antibodies and rheumatoid factor can be helpful when autoimmune neutropenia is considered, as positive results may raise suspicion for an undiagnosed collagen vascular disorder or may just support a less specific autoimmune problem. Direct antiglobulin test and antiphospholipid antibodies can also be supportive when there is suspicion for an autoimmune process. Quantitative immunoglobulin levels may reveal an underlying immunodeficiency when there is evidence of autoimmunity or when infectious complications are disproportion ate. Serologic tests for some viral infections (HIV, hepatitis, Epstein Barr virus [EBV]) may sometimes be appropriate. The main utility of flow cytometry occurs when peripheral blood smear suggests an abnormal lymphocyte population (e.g., LGLs or hairy cells). Because LGL leukemia is a relatively common cause of significant neutropenia and because this is a diagnosis frequently initially missed, one should have a low threshold to obtain flow cytometry, assuring that proper markers are analyzed (see later). BM examination may not be helpful or warranted with mild or even moderate isolated neutropenia or in straightforward cases of severe neutropenia (e.g., after drug exposure), but can be essential when the diagnosis is in doubt and especially if other cell lines are compromised.
Severe Congenital Neutropenias
It is beyond the scope of this chapter to review in detail the heterogeneous genetic disorders that manifest in early childhood as severe congenital neutropenia (SCN); nevertheless, consultant hematologists should have some familiarity with this problem both because modern therapy has extended survival for many into adulthood, and because milder forms of these disorders may go undiagnosed until young adulthood. More severe forms of SCN present in infancy with severe stomatitis and recurrent bacterial infections. The molecular bases have been greatly elucidated, among them: (1) an autosomal dominant mutation of the neutrophil elastase gene ELANE in 50% to 60% of patients, (2) an autosomal recessive mutation of HAX1, which is associated with mental retardation and other congenital defects found in Kostmann syndrome, (3) the other genes account for only a small proportion of all cases: G6PC3, GF11, CSF3R, WAS (X-linked), CXCR4, VPS45A and JAGN1, and (4) about one-third of patients do not have identifiable mutations in the above-mentioned genes.
In young children, the differential diagnosis of SCN includes transient postinfectious neutropenia, alloimmune neutropenia, or hematologic malignancy. BM examination often shows a pattern of maturation arrest (Fig. 6). The majority of affected patients are responsive to G-CSF, which has favorably impacted infectious morbidity and mortality. Allogeneic stem cell transplant is another therapeutic option. An intrinsic risk of acute myeloid leukemic transformation, 15% at 20 years, accompanies these disorders, with higher risk in those requiring higher doses and/or responding poorly to G-CSF.
Fig6. MATURATION ARREST (bone marrow aspirate). (Almost no myeloid precursors beyond myelocyte [sometimes promyelocyte] stage.) Non-specific: can be seen with early precursor release, early recovery from myelosuppression, peripheral destruction (including antibody-mediated), and stem cell disorders.
Cyclic neutropenia is a rare autosomal dominant disorder with variable expression, also caused by autosomal dominant mutations of ELANE or rarely its transcription regulator. Neutrophil counts vary from mildly to severely low with predictable periodicity, usually about 21 days. The great majority of cases are responsive to G-CSF at low dosage.
Warts, Hypogammaglobulinemia, Infections, and Myelokathexis Syndrome
WHIM syndrome is a rare autosomal dominant immunodeficiency with Warts, Hypogammaglobulinemia, Infections, and Myelokathexis. It is due to heterozygous gain-of-function mutations in the chemokine receptor, CXCR4. CXCR4 is expressed on hematopoietic progenitor cells and regulates homing to BM where the ligand of CXCR4, CXCR12 (also known as SDF-1), is abundantly expressed. In WHIM syndrome, mutant CXCR4 prolongs interaction with CXCR12 and causes excessive marrow retention of neutrophils and other leukocytes. The diagnosis is made based on DNA sequencing for relevant mutations in CXCR4. Treatment includes G-CSF (titrate to the lowest possible dose to achieve an ANC between 1000 and 2000/mm3) and more recently, CXCR4 antagonists, such as plerixafor or mavorixafor, have been used with success.
Shwachman-Diamond syndrome is an inherited BM failure dis order due to biallelic pathogenic variants in SBDS, EFL1, or SBDS, or a heterozygous pathogenic variant in DNAJC21. Neutropenia is the most common presenting cytopenia, but bilineage or trilineage cytopenia can occur. Patients have hypocellular BM and characteristic exocrine pancreatic dysfunction manifesting as diarrhea or failure to thrive. Positive family history and other constitutional abnormalities, such as short stature and skeletal malformation, can point toward this diagnosis. About 15% to 20% of patients progress to MDS/AML by the age of 20. Hematopoietic stem cell transplantation should be considered for progressive BM failure or MDS/AML.
Lower Neutrophil Count With Fy(a-b-) Status (Previously Known as Benign Ethnic Neutropenia)
Individuals of African descent may have chronic mild neutropenia not associated with infectious complications. The 1999 to 2004 National Health and Nutrition Survey found that Black individuals had mean leukocyte counts 900/mm3 lower than White individuals, with ANCs below 1500/mm3 in 4.5% (compared with 0.8% of White individuals). Low ANC is correlated with homozygosity of a single nucleotide polymorphism in the gene Duffy antigen receptor for chemokines (DARC; also known as ACKR1; rs2814778), which results in the absence of Duffy antigen from RBCs (termed Duffy null or Fy(a-b-)). The Fy(a-b-) phenotype is commonly found in individuals from sub-Saharan African and Arab, but rare in White European or Asian ancestry. BM biopsies performed for isolated low ANC in the setting of the Fy(a-b-) genotype rarely reveal abnormalities. Thus, an asymptomatic mildly neutropenic person without abnormalities on blood smear who carries the Fy(a-b-) genotype may require no further diagnostic workup. It may be reassuring to the patient (and the doctor) if prior blood counts can be retrieved to document the chronic, nonprogressive, and complication-free course of the neutropenia.
Autoimmune Neutropenia (Primary and Secondary)
Autoimmune neutropenia can be primary or secondary to an auto immune disorder such as SLE or RA. Neutropenia can be mild or severe. Classically, it is caused by antineutrophil autoantibodies, although autoreactive cytotoxic lymphocytes may also cause this problem. When performed, BM examination often is normocellular or hypercellular, with a late “maturation arrest” picture. Severe cases can display pure WBC aplasia (similar to drug-induced agranulocytosis, which may sometimes also be antibody-mediated). There are several challenges to making the diagnosis. One is that the maturation arrest BM picture is very nonspecific with regard to mechanism: it could indicate a true stem cell differentiation defect, or the early release of later precursors (as with sepsis or splenic sequestration), or an immune attack aimed at antigens on later myeloid precursors, or early recovery from a toxic insult. Another problem is the lack of clinically reliable neutrophil antibody tests. Experimentally, several methods have been used, generally suffering from high false-negative and false-positive rates in detecting antibodies against neutrophil antigens or immune complexes that bind to neutrophil Fc receptors. With no definitive test, diagnosing autoimmune neutropenia rests largely on clinical context and judgment.
Neutropenia caused by antineutrophil antibodies has been seen in infants about 1 year old, usually running a benign clinical course with spontaneous remission in 95% within 2 years. Newborns are at risk of alloimmune neutropenia in the first months of life, but this is not generally accompanied by infectious complications. Secondary autoimmune neutropenias are most common in adults, related to disorders such as SLE, RA, Sjögren syndrome, thymoma, and common variable immunodeficiency. In SLE, neutrophil counts correlate with disease activity and have been related to the induction of TNF–related apoptosis-inducing ligand (TRAIL), a ligand that increases myeloid apoptosis and killing by autologous T cells. Immunosuppressive therapy of the SLE or the use of G-CSF can improve the WBC count.
Felty syndrome was classically described as severe neutropenia and splenomegaly complicating RA, and has been attributed to antineutrophil antibodies, but the distinction between this syndrome and other causes of neutropenia with RA have become blurred. In particular, classic Felty syndrome and LGL syndrome appear to represent points on a disease continuum. About 1% of patients with RA develop Felty syndrome, and it predisposes many to serious infections. Effective therapies have often included methotrexate, gold salts, and rituximab. Because of some risk of exacerbating underlying inflammatory problems, G-CSF should be used with caution.
Large Granular Lymphocyte Leukemia
LGL leukemias are covered elsewhere, but mention is required here because these are common considerations in adults with neutropenia. LGL leukemia is a chronic lymphoproliferative disorder due to clonal expansion of cytotoxic T or NK LGLs. The expansion of cytotoxic effector cells are due to chronic antigen-driven immune stimulation and therefore treatment is based on immunosuppression. These proliferations are frequently associated with autoimmune disorders (especially RA), variable degrees of neutropenia, anemia, commonly splenomegaly, and increased infectious risks. The diagnosis is suggested on blood smear by large lymphocytes with mature chromatin, excessive cytoplasm, with or sometimes without prominent cytoplasmic granules. Flow cytometry demonstrates increased CD3+CD57+ lymphocytes with the more common T-LGL proliferations or may show CD3− CD56+ clones with “pure” NK cell LGL proliferation. Clonality can be established by T cell receptor (TCR) repertoire analysis by PCR. Killer cell immunoglobulin like receptor (KIR) repertoire analysis by flow cytometry is used to determine clonality in NK-LGL. Somatic activating mutations in the signal transducer and activator of transcription 3 gene (STAT3) were found in 40% of patients with LGL leukemia, suggesting aberrant STAT3 signaling pathway is an underlying mechanism of this disease. Somatic activating STAT5b mutations were also found in a minority of patients.
Neutropenia With Infectious Diseases
Leukocytosis is the expected response to most bacterial infections, but leukopenia also occurs and is characteristic of infection by certain organisms. Viral infections often cause transient mild leukopenia, so restraint is wise in the diagnostic approach to a newly recognized moderate leukopenia in a febrile, modestly ill patient. Nonbacterial infections in which neutropenia is frequent or even characteristic include HIV, EBV, cytomegalovirus (CMV), hepatitis A and B, measles, rubella, varicella, rickettsia, and anaplasmosis (ehrlichiosis). Bacterial infections with characteristic leukopenia include typhoid fever and brucellosis, and granulomatous infections (tuberculosis, histoplasmosis [Fig. 7] also commonly cause neutropenia, especially when the BM is directly involved. In patients with poor BM reserve (e.g., prior chemotherapy, malnutrition, myelodysplasia), an acute infection very often lowers, rather than raises, the neutrophil count. Profound neutropenia is a known consequence of overwhelming sepsis and has been associated with poor outcomes.
Fig7. HISTOPLASMOSIS. In disseminated histoplasmosis, organisms can frequently be seen in leukocytes and histiocytes in bone marrow and sometimes peripheral blood.
Hypersplenism
Hematologists are frequently consulted for cytopenias, only to find an unappreciated large spleen as the etiology (this is most commonly due to liver disease with portal hypertension). Splenic enlargement can lead to sequestration and reduction of circulating WBCs, RBCs, platelets, or any combination of these. The degree of cytopenia is somewhat proportional to the degree of splenic enlargement. Substantial neutropenia out of proportion to the depletion of other cell lines and to the degree of splenomegaly suggests an autoimmune component (which can be seen with hepatitis C or autoimmune hepatitis) or medication effect (e.g., interferon).
Chemotherapy-Induced Neutropenia
The major dose-limiting toxicity of most cancer chemotherapy regimens is neutropenia. This results in hospitalizations, antibiotic costs, reduced quality of life, infectious morbidity, dose reductions, and treatment delays that compromise efficacy and outcomes, and result in some deaths: 75% of chemotherapy-related mortality. Febrile neutropenia is defined as an ANC less than 500/mm3 (or expected to decrease to 38.3°C or sustained >38°C).
Principles of empiric antibiotic coverage for acute febrile episodes in severely neutropenic patients are covered in detail else where. Basically, there is a high risk of sepsis (even when the source is occult), and a favorable outcome depends on prompt effective antimicrobial coverage, so empiric administration of broad-spectrum antibacterial agents is considered medically emergent. There must be coverage of the traditional aerobic gram-negative rod culprits (particularly Pseudomonas), yet vigilance must be maintained for the Gram-positive organisms that have emerged as major pathogens. With prolonged or repeated episodes of neutropenia, invasive fungi become common threats to survival, particularly Aspergillus, Candida, Mucor, and Fusarium spp. Published guidelines can aid the choice of empiric antimicrobials. Because of difficulty demonstrating favorable impact, enthusiasm for granulocyte transfusions has waned in the past 30 years.
The utility of prophylactic antibiotics is problematic because of the wide array of potential pathogens and because of concerns of inducing antibiotic resistance. Prophylactic fluoroquinolones, antifungals, and antivirals have had some favorable impact in very high-risk patients after consolidation therapy for acute leukemia or allogeneic stem cell transplant.
G-CSF or pegfilgrastim is widely used for primary or secondary prevention of neutropenia to decrease morbidity and mortality of febrile neutropenia. Primary prophylaxis is recommended by various guidelines if the risk of developing febrile neutropenia is greater than 20%. This risk is calculated from age, extent of primary cancer, comorbidities, and the known myelotoxicity of the chemotherapy regimen. G-CSF is begun 24 to 72 hours after the completion of myelotoxic chemotherapy.
Drug-Induced Neutropenia
Drug reactions account for a high percentage of acquired neutropenia, both mild but, more so, severe cases (agranulocytosis; Table1). Neutropenia often develops abruptly, within 4 weeks of initiation of a causative agent. Pathophysiology may involve immune mechanisms or more direct toxicity such as enhancement of reactive oxygen species made by nicotinamide adenine dinucleotide phosphate oxidase or myeloperoxidase on neutrophil precursors. They can be strongly suspected from an accurate history that plots leukocyte numbers against the time course of drug exposure; confirmation is leukocyte recovery after drug withdrawal. More specific confirmatory testing, such as drug-dependent antibodies, is mainly an area of research laboratory investigation. BM examination is not usually required but may be helpful when pathophysiology and diagnosis are in doubt, when the course is atypical, and to provide prognostic information (earlier recovery may be anticipated with a picture of “maturation arrest” than that of myeloid aplasia). Treatment demands discontinuation of suspected offending drugs. Although the efficacy of myeloid growth factors (G-CSF) has been questioned, most recommend this therapy because even a modest shortening of severe neutropenia can occasion ally be lifesaving. Mortality of drug-induced agranulocytosis is generally reported to be 10%.
Table1. Drugs Commonly Associated With Neutropenia
The most commonly implicated drugs are listed in Table 1. Some merit additional comment. Some drugs have a direct myelotoxic effect rather than induce an idiosyncratic immune-based reaction. Neutropenia will have more gradual onset related to dose and duration, and may be accompanied by suppression of other hematologic cell lines. Linezolid and most instances of chloramphenicol myelosuppression are examples.
Beginning 2007, an epidemic of agranulocytosis was found among intravenous cocaine users. The adulterant levamisole was found in 71% of confiscated cocaine by the Drug Enforcement Agency, with the incidence of agranulocytosis 2.5% to 13% in those exposed. Nadir BM showed severe myeloid hypoplasia. Levamisole had long been linked to agranulocytosis, having been used as an antihelminth and as an immune adjuvant in patients with autoimmune disorders and colorectal cancer.
Severe neutropenia is seen in patients treated with rituximab. This neutropenia is unusual for its late onset, generally about 3 months after the last rituximab dose. It occurs with underlying autoimmune disorders, B-cell malignancies, and stem cell trans plants—basically in all situations in which rituximab is used—often occurring while the underlying illness is in complete remission. Severe neutropenia has been reported in about 5% of rituximab-treated patients, much higher in some series. Most patients recover quickly, usually after G-CSF therapy, but there have been protracted cases. The authors and others have reported a high risk of relapse (100% in our series) if rituximab is reinitiated. The mechanism is not definitively established, but an intriguing report implicates imbalanced recovery of B-cell clones with a deficiency of stromal-derived factor 1.
Immunotherapy-induced severe neutropenia has been reported in individuals with pre-existing autoimmune disorders.
