CBC Picture in Leptospirosis
The typical CBC in leptospirosis shows thrombocytopenia (present in 65-88% of cases), normal to mildly elevated white blood cell counts with neutrophilia and toxic changes, progressive decline in hemoglobin, and the presence of reactive lymphocytes—a pattern that helps differentiate it from dengue and other acute febrile illnesses. 1, 2, 3, 4
White Blood Cell Pattern
- Total WBC counts are typically normal or mildly elevated rather than markedly elevated, with 75% of patients showing normal leukocyte counts on day 3 of fever 2
- Leukocytosis (>11,000 cells/mm³) develops in only 38% by day 5 and is significantly associated with severe disease 1, 2
- Neutrophil leukocytosis occurs in 60% of cases with a characteristic left shift seen in 40% of patients 4
- Toxic changes in neutrophils are present in 70% of cases, which is a distinctive finding 4
- Lymphopenia develops in over half of patients by day 5, declining to about 25% by day 10 2
- Reactive lymphocytes appear in 70% of cases, and the combination of reactive lymphocytes with either toxic neutrophils or neutrophil leukocytosis is highly suggestive of leptospirosis 4
Platelet Pattern
- Thrombocytopenia (<150,000 cells/mm³) is the most consistent finding, occurring in 65-88% of patients 1, 2, 3
- Platelet counts decline progressively over the first 6 days of illness, with the nadir typically occurring around days 5-6 2, 3
- Mean baseline platelet counts are approximately 69 × 10⁹/L in affected patients, rising to 151 × 10⁹/L following treatment 3
- Severe thrombocytopenia is significantly associated with severe disease (p=0.002), with marked thrombocytopenia predicting worse outcomes 1, 3
- Platelet counts below 29 × 10⁹/L are associated with hemorrhagic complications, which occurred in patients who subsequently died from bleeding 3
Hemoglobin and Hematocrit Pattern
- Progressive decline in hemoglobin occurs from day 1 to day 7, with mean hemoglobin falling below 12.5 g/dL in both mild and severe disease 1
- Hemoglobin and hematocrit levels decline throughout the course of illness rather than remaining stable 2
- Significantly lower hemoglobin and hematocrit levels predict severe disease, distinguishing complicated from uncomplicated cases 2, 5
- Target cells may be seen in 50% of peripheral blood smears, adding to the diagnostic picture 4
Temporal Evolution of CBC Changes
- Leukocyte and neutrophil counts show a biphasic pattern: declining over the first 5 days, then rising until the end of the second week 2
- Thrombocytopenia is most prominent between days 3-5 of illness, making this the optimal window for detecting this finding 2
- Serial monitoring is essential as single CBC measurements may miss the characteristic patterns 1, 2
Markers of Severe Disease
- Higher total WBC and neutrophil counts from day 4-5 onward (p=0.001) indicate progression to severe disease 1, 2
- Marked thrombocytopenia with platelet counts <50 × 10⁹/L is associated with acute renal failure and multiple organ involvement 3
- Declining hemoglobin with neutrophilia predicts severe leptospirosis and helps identify patients requiring intensive monitoring 2, 5
Differential Diagnosis Clues
The combination of reactive lymphocytes with toxic neutrophils or neutrophil leukocytosis strongly suggests leptospirosis over dengue, where lymphocytosis without toxic changes is more typical 4. Unlike dengue, leptospirosis shows neutrophilia rather than lymphocytosis, and unlike bacterial sepsis, leukocytosis is less pronounced with more prominent reactive lymphocytes 2, 4.
Critical Pitfalls to Avoid
- Do not rule out leptospirosis based on normal WBC counts, as 75% have normal counts early in the disease course 2
- Do not wait for marked leukocytosis to consider the diagnosis, as this is uncommon and occurs late 2
- Do not dismiss the diagnosis if thrombocytopenia is absent, as 12-35% of patients maintain normal platelet counts 1, 2, 3
- Do not rely on a single CBC, as the temporal evolution of findings is diagnostically important 1, 2