Acute hives (lasting less than 6 weeks)

Drugs (including medications) such as

• Some opioids

• Contrast agents (used with imaging tests such as computed tomography)

• Possibly any drug or medication, whether prescription, over-the-counter, or herbal, if people are allergic to it

Hives can start within minutes after the drug was used and can start up to 48 hours later

A doctor's examination

Foods that trigger an allergic reaction (food allergens), such as peanuts, nuts, fish, shellfish, wheat, eggs, milk, and soybeans

Hives that start within minutes or hours after consumption

A doctor's examination, particularly the medical history

Sometimes allergy skin prick testing

Infections (rare causes)

• Bacterial, such as some urinary tract, streptococcal, and Helicobacter pylori infections

• Parasitic, such as Toxocara canis, Giardia intestinalis (lamblia), Schistosoma mansoni, Strongyloides stercoralis, Trichuris trichiura, and Blastocystis hominis infections

• Viral, such as hepatitis (A, B, or C) and HIV, cytomegalovirus, Epstein-Barr virus, and enterovirus infections

Fever, chills, and fatigue

Symptoms of the particular infection

Particularly for parasitic infections, recent travel to a developing country

Tests depending on the suspected infection (suggested by results of the medical history and examination)

Diagnosis confirmed if hives disappear after the infection is eliminated

Insect bites or stings

Hives that start within seconds or minutes after an insect bite or sting

A doctor's examination

Physical stimuli

• Cold

• Exercise

• Pressure on the skin

• Scratching of the skin (urticarial dermatographism)

• Heat

• Sunlight

• Sweating as occurs during a warm bath, exercise, or fever

For most stimuli, hives that typically start within seconds or minutes of exposure to the stimulus

For pressure on the skin, hives that start within 4–6 hours and affect only the area of the skin pressed

For sunlight, hives that affect only the area of the skin exposed to sunlight

A doctor's examination

Exposure to the suspected physical stimulus to see whether it triggers symptoms

Serum sickness

Hives that start within 7–10 days after injection of

• A blood product (as in a transfusion)

• A drug derived from animal blood such as horse serum (which is used to treat poisonous snake and spider bites)

• Possibly another drug

May be accompanied by fever, joint pain, swollen lymph nodes, and abdominal pain

A doctor’s examination

Substances that trigger an allergic reaction through direct contact with the skin (contact allergens), such as latex or meat

Hives that start within minutes or hours after contact

A doctor's examination

Sometimes allergy testing

Transfusion reactions

Hives that usually start within a few minutes after transfusion of a blood product

A doctor's examination

Chronic hives (lasting more than 6 weeks)

Autoimmune disorders

• Systemic lupus erythematosus (lupus)

• Sjögren syndrome

• Urticarial vasculitis

Various symptoms depending on the autoimmune disorder

For systemic lupus erythematosus, symptoms may include fever, fatigue, headache, joint pain and swelling, painful breathing, and mouth sores

For Sjögren syndrome, dry eyes and dry mouth

For urticarial vasculitis, hives that

• May be painful rather than itchy

• Usually last more than 24 hours

• Do not whiten (blanch) when pressure is applied

• Can be accompanied by small blisters and reddish purple blotches (purpura)

For all autoimmune disorders, blood tests to check for abnormal antibodies

Sometimes skin biopsy

For Sjögren syndrome, a test that estimates the amount of tears people are producing

For urticarial vasculitis, skin biopsy

Cancer, typically of the digestive organs or lungs, or lymphoma

Weight loss, night sweats, abdominal pain, cough (sometimes bringing up blood), jaundice, swollen lymph nodes, or a combination

Various tests depending on the cancer suspected

Chronic idiopathic hives (diagnosed when no specific cause is identified)

Hives that occur daily (or almost daily) and itching that lasts for at least 6 weeks, with no obvious cause

A doctor’s examination

Blood and sometimes other tests, such as skin prick tests and exposure to various triggers, to rule out other causes

Drugs, including medications (same as those for acute hives)

Hives that occur in a person who has been taking a prescription, an over-the-counter, or an herbal drug for a long time when there is no other explanation for them

A doctor's examination

Sometimes allergy testing

Trial of avoidance to see whether hives disappear after the drug is stopped

Endocrine abnormalities such as a thyroid disorder

For thyroid disorders, difficulty tolerating heat or cold, a slow or fast heart rate, and shaking (tremor) or sluggishness

A doctor's examination

If a thyroid disorder is suspected, a blood test to measure thyroid-stimulating hormone

Mastocytosis

Small red discolorations of the skin that turn into hives and become raised when touched

Sometimes abdominal pain, easy flushing, and recurring headaches

Skin and sometimes bone marrow biopsy

Sometimes blood tests to measure levels of substances released when certain immune cells (called mast cells) are activated

Physical stimuli (same as those for acute hives)

For most stimuli, hives that typically occur within seconds or minutes of exposure to the stimulus

For pressure on the skin, hives that start within 4–6 hours and affect only the area of the skin pressed

For sunlight, hives that affect only the area of the skin exposed to sunlight

A doctor's examination

Exposure to the suspected stimulus to see whether it triggers symptoms