7 Commonly Misdiagnosed Illnesses

En español | Feeling fatigued? It could be fibromyalgia or lupus.

Suffering from abdominal pain and weight loss? You could have cystic fibrosis or celiac disease.

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Some illnesses and conditions are tricky to diagnose.

Many Americans suffer from illnesses that are difficult to diagnose (see "Misdiagnosed: What to Do When Your Doctor Doesn't Know"). We reveal seven ailments that give doctors trouble, identify their symptoms and tell you how to get help if you think you have one.

1. Lupus

A chronic inflammatory disease

Symptoms: Fatigue; kidney, heart, and lung damage; rash; joint pain

Mimics: Chronic fatigue syndrome, fibromyalgia, rheumatoid arthritis

Tests: Complete blood count to assess presence of anemia and a decreased white blood cell count; chest X-ray that may show pleurisy or lupus-related pneumonia; anti-double-stranded DNA test (anti-dsDNA), currently the most specific test for lupus; ANA and lupus erythematosus cell tests, which may show positive results in active lupus.

2. Parkinson's disease

A degenerative disorder of the central nervous system

Symptoms: Tremors in hands, arms, legs, or head; stiff muscles; problems with balance or walking

Mimics: Alzheimer's, stroke, stress, traumatic head injury, essential tremor

Tests: There are no lab tests that can diagnose Parkinson's. It is typically diagnosed by a clinical examination.

Photo by BSIP/Photo Researchers

Joint pain is a symptom of many conditions.

3. Fibromyalgia

A chronic arthritis-like disorder characterized by widespread pain

Symptoms: Anxiety or depression, increased sensitivity to pain, incapacitating fatigue

Mimics: Rheumatoid arthritis, chronic fatigue syndrome

Tests: Though no diagnostic lab tests are available, a diagnosis is typically made when patients report two symptoms: (1) a history of widespread pain lasting more than three months; and (2) the presence of at least 11 tender points, spots on the body that are extraordinarily sensitive to pain.

4. Lyme disease

A systemic infection caused by a tick bite

Symptoms: Shortness of breath; chest or rib soreness; abdominal cramping, nausea, and vomiting; bull's-eye rash at bite site; neck stiffness; twitching of face or eyelids

Photo by Custom Medical Stock Photo/Getty Images

Lyme disease is spread through a bite from an infected deer tick.

Mimics: Mononucleosis, flu, chronic fatigue syndrome, fibromyalgia, depression, meningitis

Tests: If no rash is visible, a two-step blood antibody test is recommended: the ELISA or IFA blood test, followed by the more specific Western blot blood test.

5. Multiple sclerosis

A progressive autoimmune disease that attacks the central nervous system

Symptoms: Muscle spasms, lack of coordination, balance problems, blurred vision, cognitive impairment

Mimics: Viral infection, lupus, Alzheimer's, bipolar disorder

Tests: Blood tests to rule out other disorders, a lumbar puncture, and an MRI, which may show the brain and spinal cord damage and lesions characteristic of multiple sclerosis

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A diet of gluten-free grains can be used to treat celiac disease.

6. Celiac disease

An autoimmune disorder marked by an inability to digest gluten, a protein in wheat, rye, and barley

Symptoms: Vomiting, abdominal pain and bloating, diarrhea, weight loss, anemia, leg cramps

Mimics: Irritable bowel syndrome, Crohn's disease, cystic fibrosis

Tests: Blood tests can detect high levels of certain antibodies; roughly 10 percent of people with celiac disease, however, test negative for it. Plus: small-intestine-sample biopsy, genetic (DNA) tests.

7. Chronic fatigue syndrome

A complex disorder with a combination of symptoms of unknown cause

Symptoms: Loss of memory or concentration, sore throat, painful lymph nodes in neck or armpits, unexplained muscle and joint pain, extreme exhaustion

Mimics: Sinus problems, hepatitis, fibromyalgia, lupus, rheumatoid arthritis

Tests: No lab or other diagnostic procedures exist to confirm the presence of chronic fatigue syndrome. Diagnosis is based on exclusion of similar disorders.