The investigations need repeating as initial work up many times come normal.
Having said that; all of your symptoms; nausea, fever, tiredness, lack of appetite, stiff muscles are significant. A chronic viral infection is one of the foremost possibility so a complete blood count, chest x-ray, abdominal ultrasound and comprehensive metabolic panel need to done as the initial investigative work up. If all of these come out to be normal, following investigations in the step wise fashion according to physical exam finding will be recommended for the diagnosis and initiating the treatment:
1) Obtain cultures for bacteria, mycobacteria, and fungi (blood, urine, stool, sputum, spinal fluid).
2) Serologies for Brucellosis, CMV infection, amebiasis, toxoplasmosis, and chlamydial diseases.
3) Serum protein electrophoresis (SPEP) is useful in diagnosing atrial myxoma, SLE flares, etc.
4) Serum ferritin levels are useful in cases of SLE flare, and adult Still disease.
5) Antinuclear antibody titers, rheumatologic factor, thyroxine level, and ESR,
6) High resolution CT scan of lung and CT scan of chest.
7) Ultrasound and CT scan of abdomen.
8) Adrenal function tests.
9) Peripheral blood smear
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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