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Common Tests and Tools for Diagnosing a Rare Disease

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By Editorial Team

4 min read

Reviewed by: HU Medical Review Board

Incorrect or delayed diagnosis of rare diseases is common. Because so few people have a particular rare condition, doctors you go to may have never seen someone with your condition. This means large numbers of people remain undiagnosed for years. Many die without an accurate diagnosis.1,2

About 7,000 rare diseases have been identified, affecting 1 in 10 or about 30 million people in the United States. Delays in diagnosis can last for months to decades, and depend on a person's age and available resources. People with rare diseases may undergo expensive, complex workups at more than 1 institution. Even with these tests, people often go misdiagnosed or undiagnosed.1

The tests someone with a rare condition might face vary widely, depending on their symptoms and other issues. Some of the most common tools to diagnose an illness are:3-13

  • Medical and family history
  • Physical exam
  • Blood and urine tests
  • Imaging tests
  • Spinal tap
  • Endoscopy
  • Electrocardiography (ECG or EKG)
  • Electroencephalography
  • Biopsy or exploratory surgery
  • Prenatal tests, if pregnant
  • Newborn screening
  • Genetic testing

Medical history

A medical history exam is a detailed review of your health. It may be done by a doctor or other health professional. This exam usually includes questions about your current symptoms, past illnesses, family medical history, and any medicines and supplements you take. The doctor may also ask about your lifestyle and diet, such as if you smoke and how much alcohol you drink.4,6

The doctor gathers this information to help decide if you have an illness or injury that needs medical care. The doctor will also use this information to develop a plan for diagnosis and treatment.4

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Things the doctor may look for when diagnosing a rare condition might include:4

  • Pregnancy and birth history
  • Family history of disease
  • Any health changes you have noticed
  • Any injuries and surgeries
  • Frequency, timing, and duration of symptoms
  • What seems to trigger symptoms, if anything
  • Signs of multiple diseases or conditions

Depending on your age and symptoms, they may ask you about:4-6

  • Any new drugs you are taking, including over-the-counter and dietary supplements
  • Recent travel
  • History of infections
  • Sexual history
  • Response to previous treatments

Physical exam

A physical exam includes weighing you and checking your blood pressure, heart rate, and temperature. The doctor will also check other organ systems. This includes looking at your eyes and ears, tapping on certain parts of the body, and listening to your heart and breathing. This gives the doctor basic information about your overall health.4,5

Doctors are trained to look for signs of illness. Depending on the suspected condition, your doctor may look for:4-6

  • Unusually colored skin or bruising
  • Muscle tenderness
  • Swelling
  • Painful movement
  • Balance issues
  • Problems answering questions

Tests

There is rarely a single test that gives your doctor a definite answer about whether you have a rare disease. The only exception are those conditions where doctors have found a genetic mutation that causes your condition. However, this requires genetic testing. And genetic testing is rarely done early in someone's diagnostic journey.1,2,6

Doctors may perform a number of tests to rule out the most obvious health conditions. Some of the tests your doctor may perform include:4-6

  • Blood tests such as complete blood count, erythrocyte sedimentation rate, complete metabolic panel, or C-reactive protein level
  • Biopsy (surgery to gather cells and look at them under a microscope and run other tests)
  • Imaging tests such as X-ray, MRI (magnetic resonance imaging), or CT (computed tomography) scan
  • ECG

Prenatal and newborn tests

Prenatal testing occurs during pregnancy, before birth. These tests look for common birth defects or other problems in an unborn baby. If a family history of a rare disease is known, additional prenatal tests may be done.7,8

Amniocentesis

During this procedure, the doctor uses a hollow needle to take some of the fluid that surrounds a baby. This is done by inserting a hollow needle through the pregnant person's belly into the womb (uterus).9

The doctor sends the fluid sample to a lab. The lab checks it for genetic mutations that can cause inherited diseases or defects, such as hemophilia.9

Amniocentesis is not routinely done. Instead, your doctor will want to do one if they suspect your baby has a disorder or complication. When it is needed, it is usually done between weeks 15 and 20 of pregnancy.9

Chorionic villus sampling

Chorionic villus sampling (CVS) is a test that examines cells from the placenta. The cells are gathered by passing a thin tube through the cervix and into the uterus. A small amount of tissue (chorionic villi) is withdrawn using a syringe. The lab then checks the tissue sample for genetic mutations like those that cause cystic fibrosis. CVS is usually performed between weeks 11 and 14 of pregnancy.10

Cordocentesis or PUBS

Cordocentesis is also known as PUBS. PUBS stands for percutaneous umbilical blood sampling. This tests the blood of an unborn baby after the 18th week of pregnancy. PUBS is becoming a rare test since other tests may be safer for the baby.11

The doctor takes blood from the baby's umbilical vein. This blood is then sent to the lab to diagnose:11

  • Genetic disorders
  • Bleeding disorders
  • Infections

Newborn screening

Most babies undergo routine tests after they are born to screen for some of the most common and potentially devastating rare conditions. Many babies look healthy and have no family history of serious disease. So screening tests may be the only way to get early help. These tests check for certain serious conditions, including:12-14

  • Phenylketonuria
  • Spinal muscular atrophy
  • Severe combined immunodeficiency
  • Pompe disease
  • Certain heart conditions

Genetic testing

Doctors believe 8 in 10 rare diseases have a genetic cause. Genetic testing involves taking blood or tissue and looking for changes in genes known to cause a rare condition.1,14

Genetic testing has become more common as the costs have come down. It holds the promise of ending an expensive, invasive, and emotionally draining diagnostic journey.1

For more information about how your condition may be diagnosed, talk to your doctor. Staff may be available to help you navigate your medical insurance, costs, and more. You may also find help online on rare disease foundation websites and forums like the ones on this site.

A diagnosis of a rare condition may come with uncertainties and unknowns. Many people live successful and fulfilling lives despite the challenges they face.