Differential Diagnoses for Arthralgia, Myalgia, Fever, Respiratory Symptoms, and Pancytopenia

Primary Rheumatologic/Autoinflammatory Conditions

The American College of Rheumatology suggests that Adult-Onset Still's Disease (AOSD) is a leading consideration given the constellation of fever, arthralgia/myalgia, and systemic inflammation, with fever present in 95.7% of cases and arthralgia/myalgia in 75-100% of cases 1
The European League Against Rheumatism recommends recognizing the typical fever pattern in AOSD as spiking with temperature ≥39°C, often accompanied by an evanescent salmon-pink rash on trunk and proximal limbs 1, 2
Arthralgia affects 64-100% of AOSD patients, most commonly involving knees, wrists, and ankles in a symmetric polyarticular pattern, according to the American College of Rheumatology 1
Myalgia is reported in 56-84% of AOSD cases, as noted by the European League Against Rheumatism 1
The American College of Rheumatology warns that pancytopenia in AOSD should immediately alert to macrophage activation syndrome (MAS), which requires prompt immunosuppressive treatment and carries significant mortality risk 1, 2
Laboratory findings in AOSD typically show marked neutrophilic leukocytosis, elevated ESR/CRP, and hyperferritinemia, as stated by the European League Against Rheumatism 1

The Centers for Disease Control and Prevention (CDC) notes that Q fever presents with fever, myalgia, arthralgia, and pneumonia in a pattern closely matching this presentation 3
The CDC reports that most frequently reported symptoms of Q fever include fever, fatigue, chills, and myalgia, with pneumonia as an important manifestation 3
The CDC states that severe headache is characteristic of Q fever and can be misdiagnosed as migraine 3
The CDC indicates that arthralgia is a common extrapulmonary manifestation of Q fever 3
The CDC warns that pancytopenia is not typical in Q fever but can occur with severe systemic involvement 3
The World Health Organization (WHO) recommends considering travel-related infections such as malaria, dengue, enteric fever, and rickettsial diseases in any patient with fever and arthralgia who has traveled to endemic areas within the past year 4

The American College of Rheumatology suggests that polymyositis/inflammatory myositis can present with proximal muscle weakness, with difficulty standing, lifting arms, and moving, and that elevated CK levels distinguish true myositis from polymyalgia-like syndromes 5, 6
The European League Against Rheumatism notes that pancytopenia is not typical in polymyositis unless there is concurrent autoimmune disease or drug toxicity 6

The American College of Rheumatology recommends documenting fever pattern, presence of rash, and joint involvement pattern, and obtaining complete blood count with differential to characterize pancytopenia 1, 2
The European League Against Rheumatism suggests measuring inflammatory markers such as ESR, CRP, ferritin, LDH, and fibrinogen 1, 2, 6
The CDC recommends checking liver enzymes, coagulation studies, and triglycerides 1, 2
The WHO advises considering blood cultures, chest imaging, travel history, and geographic exposures to guide testing for infections such as malaria, dengue, Q fever, and rickettsial diseases 3, 4
The American College of Rheumatology recommends bone marrow examination when pancytopenia is present to evaluate for hemophagocytosis, hematologic malignancy, or disseminated infection 1
The European League Against Rheumatism suggests including morphology, flow cytometry, cytogenetics, and cultures in bone marrow examination 5, 6

The American College of Rheumatology warns not to dismiss pancytopenia as part of AOSD, as it signals MAS requiring urgent treatment 1, 2
The CDC advises not to start empiric corticosteroids before excluding infection and malignancy, as this can worsen outcomes in disseminated infections or mask leukemia 3, 4
The WHO recommends not to rely on negative monospot to exclude EBV in immunocompromised patients, and to use PCR instead 4
The CDC notes that airborne transmission of Q fever occurs, and lack of animal contact does not exclude the disease 3