Neutropenia: A High-Yield USMLE Review

Neutropenia is defined as an absolute neutrophil count below 1,500/µL. Because neutrophils are the first responders to bacterial infection, their depletion places patients at high risk for serious and rapidly progressive infections. On the exam, the critical scenario is febrile neutropenia — a medical emergency demanding immediate empiric antibiotics.

Presentation

• Recurrent or severe bacterial infections, reflecting loss of the body's first-line phagocytic defense
• Fever — in the setting of neutropenia, fever (>38.3°C once, or >38.0°C sustained for 1 hour) with an ANC <500 defines febrile neutropenia, an emergency
• Susceptibility to extracellular bacteria (S. aureus, streptococci, Enterobacteriaceae) as well as gram-negative bacteremia, Pseudomonas, and Candida in profoundly neutropenic hosts
• Often occurs alongside anemia (fatigue, pallor) and thrombocytopenia (bleeding, petechiae) when due to bone marrow failure from acute leukemia

Diagnosis

• Complete blood count with absolute neutrophil count — ANC <1,500/µL defines neutropenia; ANC <500 in a febrile patient defines febrile neutropenia
• Blood cultures — obtained immediately in febrile neutropenia before or alongside starting empiric antibiotics
• Review the medication list for culprit drugs (chemotherapy, clozapine, methimazole/PTU, carbamazepine, sulfasalazine, TMP-SMX)
• MASCC score — risk-stratifies febrile neutropenia patients to guide disposition; a score ≥21 identifies a low-risk patient (roughly under 5% risk of serious complications)

Management

• Febrile neutropenia is an emergency: obtain blood cultures and start immediate empiric broad-spectrum antibiotics — commonly cefepime or piperacillin-tazobactam (antipseudomonal beta-lactam)
• Use MASCC to gate disposition: low-risk patients (score ≥21 — no hypotension, no COPD, no dehydration, ambulatory, younger) may be considered for oral therapy (e.g., ciprofloxacin plus amoxicillin-clavulanate) with close follow-up; higher-risk patients warrant admission for IV antipseudomonal monotherapy
• Identify and withdraw offending drugs; monitor at-risk agents such as clozapine

High-yield

• Neutropenia is defined as ANC <1,500/µL; febrile neutropenia = ANC <500 plus fever (>38.3°C once or >38.0°C for 1 hour)
• High-yield drug culprits: chemotherapy, clozapine, methimazole/PTU, carbamazepine, sulfasalazine, and TMP-SMX
• Neutropenia is a phagocyte defect that predisposes to extracellular bacteria — S. aureus, streptococci, Enterobacteriaceae; antibody deficiency and complement deficiency are separate, independent immune defects that each also predispose to the same extracellular organisms
• Empiric coverage must be antipseudomonal: cefepime or piperacillin-tazobactam
• MASCC score ≥21 marks a low-risk febrile neutropenia patient eligible for outpatient oral therapy
• Monocytosis can signal recovery from neutropenia

Pitfalls

• Do not wait for a fever source or culture results before starting empiric broad-spectrum antibiotics in febrile neutropenia — delay is dangerous
• Do not default every febrile neutropenia patient to admission; MASCC ≥21 identifies low-risk patients who can be treated with oral antibiotics as outpatients
• Do not overlook the medication list — drug-induced neutropenia (clozapine, methimazole/PTU, carbamazepine, sulfasalazine, TMP-SMX) is a common and reversible cause
• Do not lump neutropenia together with antibody or complement deficiency as one host defect — they are three distinct immune defects that independently raise the risk of extracellular bacterial infection
• Neutropenia impairs the inflammatory response, so classic signs of infection may be blunted — a fever may be the only clue