In general, there are four main types of blood tests:

• Hematology tests
• Biochemistry tests
• Microbiology tests
• Serology tests

Hematology tests examine the blood to identify:

• The number and types of blood cells that are 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 measure 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:

• Sodium
• Cholesterol and other fats
• Vitamins and minerals
• Enzymes
• Hormones
• Blood gases
• Prescription drugs
• Recreational drugs
• Alcohol

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 examine blood for the presence of infectious microscopic organisms such as:

• Bacteria
• Fungi
• Viruses (in most clinical labs a serology test is used)
• Parasites

Microbiology tests include:

• 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, in which 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 (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. For some skin disorders, physicians use a process of indirect immunofluorescence in serology tests. A compound is used to make antibodies in serum glow when viewed under ultraviolet light.

A variety of blood tests may be used to determine base levels of substances in blood or to screen for or identify certain conditions. Common blood tests that are sometimes used to diagnose conditions related to skin disorders include:

• Allergy blood tests. Measure the level of allergen-specific immunoglobulin E (IgE) present in the body. IgE is a major factor in the chain of events that triggers an allergic reaction.  Allergy blood tests can be used to detect allergies that may cause skin conditions such as hives (smooth, raised pink or white bumps that appear on or beneath the skin). In addition, allergy blood tests may be used to diagnose allergies in patients whose skin disorder (e.g., eczema) makes it difficult to perform an allergy skin test. Examples of allergy blood tests include radioallergosorbent test (RAST), multiple antigen simultaneous testing (MAST), immunoassay capture test and paper radioimmunosorbent test (PRIST).
  
  
• Antinuclear antibody (ANA). Checks blood levels of antibodies that are often present in patients with certain autoimmune disorders, such as systemic lupus erythematosus (a condition that can cause a skin rash) or certain bullous (blistering) disorders. A positive ANA test result is not specific evidence of a disease. Other conditions (e.g., rheumatoid arthritis, Sjögren syndrome) and the use of certain medications may also result in a positive ANA test. Therefore, further testing is necessary to establish a specific diagnosis.
  
  
• 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, severe emotional stress or tissue damage, such as burns. It may decrease with use of certain medications or in response to certain diseases, such as lupus (a chronic inflammatory disease that can affect the skin, as well as the joints, blood, kidneys and a various other body parts). In addition, a low WBC indicates that a patient may be at higher risk for infections.
  
  
• C-reactive protein test. A nonspecific test for detecting general inflammation, which occurs in skin conditions such as hives and psoriasis (a common skin disorder marked by raised, inflamed lesions that join together to form patches with distinct borders on the skin). An elevated level of this protein produced by the liver suggests an inflammatory disease such as rheumatoid arthritis.
  
  
• Complement. Measures the blood levels of a group of proteins (complement) that helps destroy foreign substances that enter the body. Patients with hives or angioedema (a condition characterized by swelling and the formation of welts [raised marks] on the skin) often show a deficiency in blood complement. Complement levels also can help a physician determine whether a case of angioedema is related to an allergy or to hereditary factors. This is important, because treatments differ for the two conditions. This type of blood test is also useful in evaluating kidney damage and in monitoring the progression of a disease.

  

• 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.
  
  CBC results can help diagnose many skin-related conditions. For example, high WBC levels may indicate tissue damage caused by a burn. CBC testing can also help reveal the presence of conditions that may cause skin rashes, such as systemic lupus erythematosus or allergies. In some cases, general itching (pruritus) is the result of a disease that can be detected with a CBC, such as hives, scleroderma and shingles (a painful rash caused by the varicella-zoster virus [VZV], the same virus that causes chickenpox).
  
  
• Enzyme test. A group of blood tests that measure enzyme (proteins required for chemical reactions to take place in cells) levels in the blood. These tests assess how well the body’s systems are functioning and whether any tissue damage has occurred. Enzyme testing may be used to detect conditions that cause skin rashes or other skin disorders. Such conditions include lupus, sarcoidosis (a disease characterized by inflammatory lumps [granulomas] that may form in many organs) and scleroderma.
  
  
• Glucose test. Measures the amount of sugar (glucose) in a patient’s blood sample. Glucose levels are mostly measured to diagnose diabetes (which is associated with slow-healing skin infections) and to monitor treatment for the blood sugar disorder. In very rare cases, greater than normal levels (hyperglycemia) may indicate acromegaly, a painful metabolic disorder that causes enlargement of a patient’s bones.
  
  
• Rheumatoid factor test. Detects the presence of rheumatoid factor (RF), an antibody found in the blood of most patients with rheumatoid arthritis. RF may also be found in other autoimmune conditions such as Sjögren syndrome (a disorder of the immune system that sometimes causes a skin rash or dry skin) and sometimes in healthy people.
  
  
• 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 typical of many forms of arthritis (e.g., rheumatoid arthritis) and many of the connective tissue diseases (e.g., lupus). Sed rate tests sometimes are used in patients who have hives, to ensure that an underlying disease is not the source of the condition.
  
  
• Thyroid stimulating hormone (TSH) test. Measures the amount of thyroid stimulating hormone in the blood. Abnormal thyroid function can be the cause of some kinds of hair loss or dry skin conditions.
  
  
• 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). When the kidneys are not working properly, waste products can build up in the body. The results of these tests can be used during the diagnosis of several diseases, including gout (a painful form of arthritis that can develop when blood levels of uric acid are chronically high) and lupus, as well as to monitor for underlying kidney disease.