I am referred occasional otherwise healthy patients for evaluation of lymphopenia (aka lymphocytopenia).
What to do?
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1. Let's begin with definitions - lymphopenia is usually defined as an absolute lymphocyte count ALC <1000 cells/microL for adults
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2. Epidemiology - lymphopenia has been documented in 1.5-3% of CBCs from both community and hospitalized patients.
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3. Causes/associated conditions - lymphopenia may be found incidentally in someone without obvious underlying cause (most common) or may be associated with a number of diseases/conditions.
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4. Mechanisms - reduced production, increased destruction, increased apoptosis, redistribution of lymphocytes between blood and various lymphoid tissues.
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5. Clinical implications - no evidence (outside of large population studies that do not take into account underlying cause) that lymphopenia associated with increased risk of infection.
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6. History and physical - focus on symptoms/signs associated with underlying cause/associated conditions(s).
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7. Lab tests - generally not needed in an otherwise healthy individual.
8. Treatment - directed towards the underlying cause.
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I tweeted a poll asking for the next step in a patient with thrombocytopenia and a platelet clumping on a peripheral smear. 62% of you answered correctly, namely to repeat the CBC in a green top (heparin-containing) tube.
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Key points:
1) Pseudothrombocytopenia is mediated by EDTA-dependent antibodies that react with platelets in blood that is anticoagulated with EDTA (chelates calcium, necessary for clotting reactions), causing platelet clumping and falsely low platelet counts.
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2) Pseudothrombocytopenia is purely an in vitro artifact.
3) EDTA-containing collection tubes are preferred for CBCs/peripheral smears because, compared with other anticoagulants, EDTA allows the best preservation of cellular components and blood cell morphology
a. Used to calculate the Hct (Hct = MCV x RBC count)
b. Used by some to predict iron deficiency vs. thalassemia (e.g., Meltzer index)
b. Largely ignored because it says nothing about the size/Hbization of RBCs
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... you could have LOTS of SMALL RBCs or FEWER LARGE RBCs amounting to the same Hct (viscosity) and Hb (oxygen carrying capacity)!
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MCV
a. Ah, now we're talking 😀! MCV is super helpful because it is the first branch point in the ddx of anemia - microcytic vs. normocytic vs. macrocytic. Each has its own specific causes.