Primary Thrombocythemia: This condition is characterized by the overproduction of platelets in the bone marrow, often leading to elevated platelet counts. A low hematocrit (Hct) might be seen due to a relative decrease in red blood cell mass compared to the increased platelet production, or it could be a separate issue such as anemia. However, primary thrombocythemia is a myeloproliferative neoplasm that can present with thrombocytosis (elevated platelets) and variable effects on other blood cell lines.

Reactive Thrombocytosis: This is a common condition where the platelet count is elevated in response to another underlying condition such as inflammation, infection, or iron deficiency anemia. The low Hct could be due to the underlying cause of the reactive thrombocytosis, such as anemia of chronic disease.
Chronic Infection or Inflammation: Conditions like tuberculosis or chronic rheumatologic diseases can lead to elevated platelet counts as part of the body's inflammatory response. Anemia of chronic disease could explain the low Hct.
Iron Deficiency Anemia: This is a common cause of low Hct and can also lead to a reactive increase in platelet production, resulting in thrombocytosis.

Polycythemia Vera (PV): Although PV typically presents with an elevated Hct due to increased red blood cell mass, early stages or certain variants might present with a relatively low Hct and significantly elevated platelets. Missing PV could lead to serious complications, including thrombosis.
Essential Thrombocythemia with Anemia: Similar to primary thrombocythemia, but with a focus on the thrombocytosis aspect. The anemia could be a separate issue or related to the underlying myeloproliferative neoplasm.
Myelodysplastic Syndromes (MDS): Certain types of MDS can present with cytopenias (low blood cell counts) in some lineages and cytosis (elevated counts) in others, including thrombocytosis. MDS can progress to acute leukemia, making it crucial not to miss.

Thrombocythemia Associated with Other Myeloproliferative Neoplasms: Conditions like chronic myeloid leukemia (CML) or myeloproliferative neoplasm, unclassifiable, can present with elevated platelet counts and variable effects on other blood cell lines.
Paroxysmal Nocturnal Hemoglobinuria (PNH): A rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells, activation of the coagulation system, and impaired bone marrow function. It could potentially present with a low Hct and elevated platelets, although this is less common.
Congenital Thrombocytosis: Extremely rare conditions that lead to inherited thrombocytosis, which might be associated with other abnormalities, including anemia.