Blood tests are useful tools for uncovering disorders in the human body, including neurological abnormalities. Blood tests may be ordered to screen for a disease before symptoms appear, help determine a diagnosis, monitor the progression of a disease and monitor the effectiveness of a patient’s treatment.
Common blood tests include:
- Complete blood count (CBC). Measures the number of red blood cells, white blood cells (WBC), hemoglobin, hematocrit and platelets. When a CBC is ordered with a differential, there is an evaluation of the different types of WBCs in the blood.
- C-reactive protein test and sedimentation rate. Detects general inflammation.
Blood tests are performed routinely to diagnose or monitor various neurological conditions. For example, blood tests can reveal signs of brain or spinal cord infection that leads to illnesses such as encephalitis and meningitis. Many other neurological conditions or symptoms require blood tests before a definitive diagnosis can be made.
Blood test results are always evaluated in relation to the “normal range” for that test. The range of values considered to be normal is based on the range of test results from the blood of normal, active healthy people.
Most blood tests require little or no preparation, and patients can generally return to their usual daily activities afterward. Additional diagnostic tests such as urine tests, imaging tests or biopsies may be necessary to confirm a diagnosis.
About blood tests
A blood test is a tool a physician uses to diagnose and/or rule out certain illnesses. During these tests, a sample of blood is drawn from a patient and collected in a tube. The blood sample is then analyzed to look for various substances that might indicate illness. Abnormally high or low levels of certain substances often indicate the presence of some type of illness.
Blood tests serve a number of purposes related to the diagnosis and monitoring of various conditions or diseases. They may be used to:
- Determine a person’s risk of developing a disease
- Screen for a disease before signs and symptoms appear
- Help determine a diagnosis
- Rule out other diseases during diagnosis
- Monitor the progression of a disease
- Monitor the effectiveness of a patient’s treatment
- Detect complications of a condition
- Detect the recurrence of a disease
- Detect the spread of the disease to other areas of the body
Although blood tests may help detect many diseases, additional tests are usually necessary to verify the diagnosis.
Samples of blood may be taken in one of three ways:
- Vein puncture (venipuncture). A needle is inserted directly into a patient’s vein to draw blood into one or more tubes for laboratory analysis. Blood is usually drawn from a vein in the inner arm, opposite the elbow.
- Skin puncture. Used when only a small amount of blood is needed to perform the desired blood test. The skin of the finger, earlobe or heel (in infants) is pricked with a needle to produce a drop of blood that is then collected for analysis.
- Artery puncture (arterial puncture). Used only when the blood test needs to be performed on a sample of oxygen-rich blood traveling from the heart to the body via the arteries. Blood is usually drawn from an artery in the side of a wrist.
Blood tests can be performed on different parts of the blood, including:
- Whole blood (blood that has not been separated into its many components)
- Blood plasma (the liquid part of the blood)
- Blood serum (plasma that has had the clotting agent removed)
- Blood cells (the individual red blood cells, white blood cells and platelets)
Types and differences of blood tests
In general, there are four main types of blood tests:
- Hematology tests. Identifies the number and types of blood cells present (e.g., red blood cells, white blood cells and platelets), the appearance of the cells (especially their maturity), the ability of the blood to form a clot and the speed at which clotting occurs.
- Biochemistry tests. Measures the levels of normally occurring chemicals and biochemicals in the blood, both individually and in relation to other chemicals. These measurements are compared to normal ranges and are used to determine whether blood biochemicals are in proper balance. Biochemicals and other substances that may be analyzed include:
- Cholesterol and other fats
- Vitamins and minerals
- Blood gases
- Prescription drugs
- Illegal/recreational drugs
Biochemical tests not only accurately measure these substances, but they can also be used to indicate how well some organs and systems are functioning. Measuring the amount of certain enzymes in the bloodstream can indirectly reflect the progression of a disease and its effect on the body.
- Microbiology tests. Examines blood for the presence of infectious microscopic organisms such as bacteria, fungi, parasites and viruses (in most clinical labs a serology test is used). Some microbiology tests are smears, in which a small amount of blood is placed on a glass slide for examination under a microscope. Sometimes the blood smear is stained with special dyes before examination.
Blood cultures are another form of microbiology test. In these tests, a small amount of blood is placed in a nutrient broth, incubated for days or weeks and then examined for growth of disease-causing bacteria.
- Serology tests. These tests performed on blood serum can detect the presence of antibodies that are produced by white blood cells to attack microscopic organisms. They are frequently used to detect viral diseases.
Common blood tests
A variety of blood tests are used to determine base levels of substances in the blood or to screen for or identify certain neurological and non-neurological conditions. Common blood tests include:
- Complete blood count (CBC). Measures the number of red blood cells, white blood cells and platelets in a patient’s blood as well as the amount of hemoglobin (a substance that carries oxygen throughout the body) in the red blood cells and a number of other factors.
- Blood differential. Also called white blood cell count (WBC). Measures the numbers of white blood cells in a blood sample. This test also includes information about abnormal cell structure and the presence of immature cells. A patient’s WBC may increase as a result of infection or decrease with use of certain medications or in response to certain diseases.
- Basic metabolic panel (BMP). A group of tests that are often ordered in hospital emergency rooms because their components provide important information about a patient’s kidneys, electrolytes and acid/base balance, in addition to glucose (blood sugar) levels.
- Antinuclear antibody test (ANA test). Checks blood levels of antibodies that are often present in patients with certain autoimmune disorders, such as lupus. Other conditions (including rheumatoid arthritis, Sjogren’s syndrome, polymyositis and scleroderma) and the use of certain medications may also result in a positive ANA test.
- C-reactive protein test. A nonspecific test for detecting general inflammation.
- Electrolyte panel. Measures various minerals, abnormal levels of which can suggest conditions ranging from cancer to kidney disease. For example, patients suspected of having ataxia may be tested for the presence of electrolytes such as sodium, potassium, chloride and bicarbonate.
- Enzyme test. A group of blood tests that measure enzyme (proteins required for chemical reactions in cells) levels in the blood. These tests assess how well the body’s systems are functioning and whether any tissue damage has occurred.
- Lyme serology. A group of blood tests that confirm a diagnosis of Lyme disease. These tests detect a patient’s immune response to the infectious agent that causes the inflammatory disease.
- Sedimentation rate (sed rate). This test measures the rate at which red blood cells (erythrocytes) separate from the liquid part of blood (plasma) and fall to the bottom of a test tube, forming sediment. Higher sed rates indicate the presence of inflammation and are sometimes used to diagnose or monitor neurological conditions.
- Serum myoglobin. Determines damage to muscle, such as from a traumatic injury, heart attack, or muscle disorders, such as polymyositis.
- Lipid profile (cholesterol test). A group of tests that measure total cholesterol, “good” HDL cholesterol, “bad” LDL cholesterol, “very bad” VLDL cholesterol and triglycerides. These tests are good indicators of a patient’s risk of heart attack or stroke.
- Tumor marker tests. Detect substances that may be produced by cancer cells, such as prostate specific antigen (PSA).
- Waste product test. A group of blood tests that measure the levels of specific waste products in a patient’s blood sample. They commonly test for uric acid, creatinine and blood urea nitrogen (BUN).
- Genetic tests. Screen for genetic abnormalities that may indicate a risk of certain diseases, such as Huntington’s disease.
Neuro disorders diagnosed with blood tests
Certain blood tests are routinely performed to diagnose or monitor various neurological conditions. For example, blood tests can reveal signs of brain or spinal cord infection that leads to illnesses such as encephalitis, meningitis and transverse myelitis.
Blood tests can reveal the presence of certain substances known to be associated with specific neurological abnormalities. For example, a blood test that reveals elevated levels of copper may be a good indicator of Wilson’s disease. Meanwhile, blood tests may indicate the presence of myasthenia gravis if they find antibodies (immune response chemicals) to acetylcholine (a neurotransmitter involved in movement) and its receptors in a patient’s muscle tissue.
Genetic testing from a blood sample can help reveal the presence of certain illnesses such as Huntington’s disease, cystic fibrosis or myotonic dystrophy. Chemical or metabolic testing of the blood can reveal protein disorders and some forms of muscular dystrophy. For example, high levels of proteins such as aldolase, creatine kinase and myoglobin often indicate muscle damage before physical signs of muscular dystrophy appear.
Blood tests also can be used to monitor the levels of certain medications used to treat neurological conditions, such as drugs used to treat epilepsy. Blood tests may also be used to identify the cause of certain neurological symptoms, such as ataxia, fainting and tremors.
Other neurological abnormalities detected or monitored through blood tests include:
- Alzheimer’s disease
- Autoimmune disorders
- Multiple system atrophy (MSA)
- Neurocutaneous syndromes
- Post-polio syndrome
- Progressive bulbar palsy
- Pseudobulbar palsy
- Primary lateral sclerosis
- Progressive muscular atrophy
- Sleep disorders
- Spinal muscular atrophy
- Sydenham’s chorea (a movement disorder associated with rheumatic fever)
Some neurological conditions (e.g., Parkinson’s disease) cannot be diagnosed through blood tests. However, blood tests are often performed to rule out the presence of other conditions that may cause symptoms similar to those associated with various neurological disorders. For example, blood testing can rule out muscle diseases and other disorders that have symptoms similar to motor neuron diseases such as ALS. Blood tests may also help reveal other causes in patients suspected of neurological conditions such as neuropathy, cranial neuralgia, Guillain-Barre syndrome or periodic limb movement disorder.
Risk factors associated with certain neurological disorders can also be revealed through a blood test. For example, a complete blood count (CBC) and a lipid profile (cholesterol test) can help a physician detect the presence of risk factors associated with stroke, such as high cholesterol. The physician can then design a long-term therapy that reduces the risk of stroke.
Before, during and after blood tests
Preparation for blood tests depends on the requirements of each specific test. Physicians will advise patients beforehand on the preparations required for each type of blood test.
In most cases, minimal preparation is necessary. The patient may be asked to reduce or stop the use of certain medications and/or vitamins at some point prior to the test. Additionally, food intake and exercise may be temporarily restricted or suspended, such as an overnight fast for some glucose tests. Consuming certain foods or beverages or taking certain drugs can skew the results of some tests. Alcohol and caffeine should also be avoided prior to a blood test.
On the day of the test, the procedure will be explained and patients will have the opportunity to ask questions. A physician or medical professional will also ask questions about the patient’s medical history prior to administering the test to determine if the patient is taking any medications that will interfere with the test’s accuracy or has any history of clotting problems.
Having blood drawn for a blood test is an easy and relatively painless process for most people. If the test only requires a drop or two of blood, then a prick of the finger, earlobe or heel (in the case of infants) can provide enough blood for testing.
A sterile, sharp lancet is used for pricking the patient’s skin. Then, the technician gently squeezes the punctured area to extract drops of blood that are collected in tiny glass tubes. To stop the bleeding, sterile gauze is applied with light pressure to the punctured area. A bandage is usually not necessary since bleeding is minimal in most cases.
When a substantial amount of blood is required, it is usually drawn from a vein in a process called venipuncture. A needle is inserted into a vein, usually at the end of the elbow or on the back of the hand. The technician cleans the area around the puncture with alcohol and a wide elastic band or piece of latex tubing may be placed around the upper arm to slightly increase pressure in the vein. One end of a sterile, double–ended needle that has been attached to an open–ended syringe (which contains an empty test tube) is then inserted into the vein. Because the test tube contains a partial vacuum, blood flows directly from the vein through the double–ended needle and into the test tube.
The precise amount of blood to be drawn is determined by the type and number of tests scheduled. After the necessary amount of blood is drawn, the needle is withdrawn and a small cotton ball or pad is applied with light pressure over the puncture site. After several minutes, the cotton will be discarded or replaced, and a small bandage will be placed on the puncture wound. The whole process usually takes less than 10 minutes.
If blood must be taken from an artery instead of a vein, it is usually drawn from a very small artery located on the inside or the top side of the wrist.
Following the withdrawal of blood in a physician’s office or laboratory, patients may resume medications and food intake according to their physician’s orders. Immediately after the blood sample is taken, the test tubes are labeled with the date and the patient’s name, and they are sent to the laboratory for testing. Results are usually returned within 24 hours, depending on the tests performed. Based on the test results, additional blood tests may be ordered.
Additional diagnostic tests such as urine tests, imaging tests or biopsies may be necessary to confirm a diagnosis.
Understanding blood test results
Blood test results are always evaluated in relation to the “normal range” for that test. The range of values considered to be normal is based on the range of test results from the blood of normal, active healthy people. Patients with a disease or health problem may have blood test results that are higher or lower than normal – or “outside of the normal range.” When a physician sees that a blood test is outside of normal range, the tests may be repeated to verify results or additional tests may be ordered to determine the underlying cause(s) behind the abnormality.
Normal ranges for some tests may vary slightly from lab to lab, especially between labs that use machines to perform blood tests and those in which blood tests are performed by hand.
Potential risks of blood tests
There are very few risks or side effects associated with blood tests. Sometimes, bruising may occur at the site of injection. This can be minimized and alleviated by keeping direct pressure on the spot for several minutes after the syringe has been removed. If soreness is present, patients can immediately apply a warm compress to the puncture site and repeat the application every three hours until the discoloration or pain subsides.
In rare cases, the vein used to obtain the sample may become inflamed. Applying warm compresses to the spot several times daily can help reduce swelling.
People with bleeding disorders or who are taking anticoagulants, including aspirin, may have problems with continued bleeding after blood tests. Those who experience these side effects should consult their physician.
Questions for your doctor regarding blood tests
Preparing questions in advance can help patients to have more meaningful discussions with health care professionals regarding their conditions. Patients may wish to ask their doctor the following questions related to blood tests:
- Why is a blood test being recommended for me?
- Which type or types of blood testing will I have?
- What can this testing tell me about my neurological condition?
- How should I prepare for the test?
- Are there any risks associated with my test?
- How long will I have to wait for results?
- What do my test results show?
- Will I need additional tests to confirm the diagnosis?
- Do I need regular blood testing to monitor my condition or medication?
- If my blood work shows I need treatment, what are my options?