A 60-year-old man comes to the emergency department due to 10 days of worsening cough, shortness of breath, and right-sided chest pain. The patient also has had low-grade fever, fatigue, and unspecified weight loss. A month ago, he was hospitalized for 2 days due to right lower lobe pneumonia. Although oral antibiotics were prescribed at discharge, he did not get the prescription filled because his symptoms had improved. Temperature is 38.2 C (100.8 F), blood pressure is 116/70 mm Hg, pulse is 104/min, and respirations are 18/min. Physical examination is notable for decreased breath sounds at the right lung base. Chest imaging reveals loculated pleural fluid on the right side. Laboratory analysis reveals the following: Blood cell counts were normal a month ago at the time of hospital discharge. Which of the following is the most likely underlying cause of this patient's increased platelet count?

Question

A 60-year-old man comes to the emergency department due to 10 days of worsening cough, shortness of breath, and right-sided chest pain.  The patient also has had low-grade fever, fatigue, and unspecified weight loss.  A month ago, he was hospitalized for 2 days due to right lower lobe pneumonia.  Although oral antibiotics were prescribed at discharge, he did not get the prescription filled because his symptoms had improved.  Temperature is 38.2 C (100.8 F), blood pressure is 116/70 mm Hg, pulse is 104/min, and respirations are 18/min.  Physical examination is notable for decreased breath sounds at the right lung base.  Chest imaging reveals loculated pleural fluid on the right side.  Laboratory analysis reveals the following:  Blood cell counts were normal a month ago at the time of hospital discharge.  Which of the following is the most likely underlying cause of this patient's increased platelet count?

Quizplus · Accepted Answer

This patient with partially treated pneumonia developed progressive pulmonary symptoms and pleural effusion indicating likely empyema. Although laboratory results were normal when he was discharged a month ago, his complete blood cell count now reveals signs of ongoing infection (neutrophilic leukocytosis) and inflammation (anemia, thrombocytosis). Infections are associated with the release of inflammatory cytokines (eg, IL-6, TNF-alpha) from activated immune cells. Elevated circulating concentrations of IL-6 stimulate the liver to increase production of thrombopoietin, which triggers megakaryocyte proliferation and maturation in the bone marrow. The subsequent increase in serum platelet count is called reactive thrombocytosis because it is a secondary process that typically resolves when the infection is adequately treated. Reactive thrombocytosis is seen in a wide range of inflammatory (eg, malignancy, rheumatologic disease, burns) and noninflammatory (eg, iron deficiency, hemolysis, blood loss, postsplenectomy) conditions and is the most common cause of elevated platelet count in all age groups. Elevated circulating levels of inflammatory cytokines also trigger the liver to generate hepcidin, which down-regulates iron absorption in the gut and iron release by the reticuloendothelial system; this frequently leads to concurrent anemia of chronic disease (normocytic anemia) due to reduced iron availability for reticulocytosis. (Choices A and E) Essential thrombocythemia, a myeloproliferative disorder characterized by the autonomous clonal proliferation of megakaryocytes, is marked by thrombocytosis, vasomotor symptoms (eg, dizziness), and increased risk of thrombosis. Primary myelofibrosis is a chronic myeloproliferative disorder characterized by significant extramedullary hematopoiesis (eg, hepatosplenomegaly); it can occasionally be associated with thrombocytosis. It is quite unlikely that this patient developed a myeloproliferative disorder in the month since he was admitted to the hospital; it is far more likely that he has reactive thrombocytosis due to his ongoing infection. (Choice B) Chronic hypoxemia (eg, chronic obstructive pulmonary disease) can trigger the release of erythropoietin from interstitial cells in the kidneys, leading to increased erythrocytosis and subsequent polycythemia. This would not significantly affect platelet count. (Choice D) Patients with congestive splenomegaly often have thrombocytosis due to reduced platelet sequestration in the spleen. However, it is far more likely that this patient has reactive thrombocytosis due to empyema. Educational objective: Reactive thrombocytosis is the most common cause of elevated platelet count in all age groups. It is generally caused by high levels of inflammatory cytokines (eg, IL-6), which prompt the liver to release thrombopoietin. Reactive thrombocytosis is seen in chronic infection, rheumatologic disease, and burns; it can also occur with hemolysis and iron deficiency anemia.

A 60-Year-Old Man Comes to the Emergency Department Due to 10