Here are the conditions most commonly misdiagnosed…

As the recent outbreak of fungal meningitis has so clearly shown, certain medical conditions are notoriously difficult to diagnose. The first of these patients suffered vague symptoms—including headache, fever, nausea and stiffness of the neck—that were initially misdiagnosed. Fortunately, doctors have now identified the tainted medication that caused the outbreak.

But mysterious symptoms are not always unraveled so quickly.

Startling statistic: Every year in the US, an estimated 40,000 to 80,000 hospital deaths are caused by diagnostic errors, according to a report in The Journal of the American Medical Association. When researchers use autopsies to discover discrepancies between diagnosed and actual causes of deaths, the error rate can be as high as 40%.

Conditions often misdiagnosed…

ALZHEIMER’S DISEASE. It’s impossible to diagnose this condition with 100% certainty because the only definitive “test” is an autopsy of the patient’s brain after death. Even though there are fairly accurate ways to determine that a patient might have Alzheimer’s (see below), mistakes are common.

Examples: Depression is one of the most common causes of Alzheimer’s-like symptoms, but doctors often fail to recognize it. Other problems, including nutritional deficiencies and medication side effects—for example, from anticholinergic drugs, such as antihistamines, incontinence medications and tricyclic antidepressants—also can cause symptoms that mimic Alzheimer’s.

Surprising fact: It’s estimated that 10% to 25% of patients with symptoms of dementia (such as memory problems and/or peculiar behavior) may have a non-Alzheimer’s condition that could be reversed with proper treatment.

What to do: Don’t accept a diagnosis of Alzheimer’s disease after a single office visit or after taking a simple questionnaire. Specialists (such as neurologists) take a very detailed personal and family history…conduct neurological and mental-status tests…and order a variety of blood and imaging tests to determine whether other conditions might be involved.

DEEP VEIN THROMBOSIS (DVT). A blood clot anywhere in the vascular system can be deadly. Those that form in the deep veins in the legs are particularly risky because the symptoms—if there are any—can seem minor. For that reason, some people don’t even seek medical care, or doctors may assume that the symptoms are caused by a leg strain or sprain.

The risk: A clot can break free and enter a lung, creating a deadly pulmonary embolism. About 20% of DVT patients who develop pulmonary embolism will die from it.

What to do: If you have leg pain, leg cramps or a sense of tightness in one leg that you can’t explain (discomfort in both legs probably is not caused by DVT), speak to your doctor right away. If he/she is unavailable, go to an urgent-care center or hospital emergency department. It’s particularly important that you get medical attention if you have DVT symptoms and are at increased risk for the condition due to cardiovascular risk factors, such as smoking or high blood pressure.

You are also at higher risk for DVT for at least three months after knee/hip replacement or if you’ve recently been immobile for hours at a time, as may occur on a long airplane flight. Other DVT symptoms may include swelling, tenderness or a reddish or bluish tint on part of the leg. DVT is easy to diagnose with an ultrasound or a CT or MRI scan.

Important: Call 911 if you have any symptoms of a pulmonary embolism—such as sudden shortness of breath or sudden, sharp chest pain that may worsen when you breathe deeply or cough.

HYPOTHYROIDISM. Patients who produce too little thyroid hormone (hypothyroidism) may have the condition for years before it is diagnosed because symptoms are usually vague and seemingly minor.

Common scenario: A doctor might assume that a patient who complains of fatigue, recent weight gain or apathy is suffering from stress or depression and write a prescription for an antidepressant.

What to do: Insist on a blood test to check your thyroid hormone levels if you have any of the above symptoms. Fatigue that’s accompanied by an increased sensitivity to cold often is a sign of hypothyroidism. So is hair loss (but not that due to male-pattern baldness). For unknown reasons, thinning of the outer one-third of the eyebrows is also a red flag for hypothyroidism.

Experts disagree on the optimal range for thyroid stimulating hormone (TSH). Current guidelines suggest that it should fall somewhere between 0.45 mIU/L and 4.49 mIU/L.(The specific values will depend on the laboratory that your doctor uses.) If your TSH is normal but your symptoms persist, ask your doctor about other blood tests, such as free T3/T4 or anti-thyroglobulin. In some patients, these tests are useful in detecting hypothyroidism.

Most people do well with a thyroid replacement regimen. Some benefit from levothyroxine (Levoxyl, Levothroid, Synthroid, etc.), while others find that natural desiccated thyroid hormone, such as Armour Thyroid, Nature-Throid or Westhroid, provides a better balance of T3 and T4 hormones.

CELIAC DISEASE. More than three million Americans have this intestinal disease in which the immune system reacts to a protein (gluten) found in wheat, barley and rye.

Because this used to be considered a rare disease, physicians—particularly older doctors who went to medical school decades ago—don’t always look for it. In addition, some of the most common symptoms, such as fatigue, abdominal pain, anemia and headaches, can be caused by dozens of other conditions.

What to do: Get tested. A blood test that looks for abnormal immune activity and other factors, perhaps followed by an intestinal biopsy, can detect celiac disease. If you test positive, you’ll need to follow a gluten-free diet. Web sites such as list foods/products that are gluten-free. Patients who eliminate all gluten will usually make a full recovery.

Don’t eliminate gluten before testing—the partial recovery of the intestinal lining will make it harder to get an accurate diagnosis.

LYME DISEASE. If a person has Lyme disease and it’s missed, he/she may go on to develop joint inflammation, heart-rhythm disturbances and even problems with concentration and memory.

Lyme disease, the most common tick-borne disease, now has been reported in most US states, but doctors often do not look for it outside the Northeast, where the disease originated. The initial symptoms, such as muscle aches and fatigue, are often confused with the flu, particularly when patients don’t even know that they were bitten.

What to do: Look for a rash. About three-quarters of patients will develop a “bull’s eye” rash along with the flulike symptoms that last for weeks. Sadly, there are no symptoms that are especially diagnostic for Lyme if the rash is missing. In these cases, symptoms may include stiff neck, headache, swollen glands, Bell’s palsy (usually temporary facial paralysis), sore throat, fever and/or tingling or numbness in the extremities.

Lyme disease can be diagnosed with a blood test. Patients who are treated with oral antibiotics at the onset of symptoms almost always make a full recovery. The longer someone goes untreated for Lyme, however, the more challenging the course of recovery may be.