Pulmonary Embolism (PE) with a false negative ultrasound: Although the ultrasound was negative, a high D-dimer level can still indicate a PE, especially if the ultrasound was not a CT pulmonary angiogram (CTPA), which is more sensitive for detecting PEs. A negative D-dimer would have ruled out PE, but an elevated level, even with a negative ultrasound, warrants further investigation.

Deep Vein Thrombosis (DVT) in an unusual location: The ultrasound might have missed a DVT in a less common location, such as the upper limbs or the pelvic veins.
Recent trauma or surgery: Recent trauma or surgery can elevate D-dimer levels due to the body's inflammatory response and clotting activation.
Malignancy: Certain types of cancer can increase D-dimer levels due to the production of pro-coagulant substances by tumor cells.
Infection or sepsis: Severe infections can lead to elevated D-dimer levels as part of the systemic inflammatory response.

Disseminated Intravascular Coagulation (DIC): A condition characterized by both widespread clotting and bleeding in the vascular system, which can be life-threatening if not promptly recognized and treated.
Aortic dissection: Although less common, an aortic dissection can present with elevated D-dimer levels and would be catastrophic if missed.
Severe COVID-19: In the context of the pandemic, COVID-19 can cause elevated D-dimer levels due to its pro-thrombotic effects.

Thyroid disorders: Certain thyroid conditions, such as thyrotoxicosis, can rarely cause elevated D-dimer levels.
Chronic inflammatory diseases: Conditions like rheumatoid arthritis or lupus can occasionally lead to elevated D-dimer levels due to chronic inflammation.
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, which can elevate D-dimer levels.