A distinction is made between acute hives and chronic hives. Acute hives last for less than 6 weeks, and chronic hives last for longer than 6 weeks. The designation of 6 weeks as the divider is arbitrary.
Most investigators believe that acute hives occur as an allergic response to ingesting a food or drug. However, in 70% of cases, the cause is not found. Acute hives usually resolve on their own. Extensive laboratory investigations are not necessary and do not yield much information. Acute hives are treated with antihistamines, which work by blocking the release of histamine from mast cells. It is important to take the antihistamines regularly because once the histamine is released, they have no affect. Over-the-counter antihistamines work well but have the side effect of sedation. The newer prescription antihistamines are also effective and do not cause drowsiness.
Hives that occur at least twice a week and have been present for more than 6 weeks are designated chronic. Unlike acute hives, chronic hives do not resolve quickly. In one study, 75% of people with chronic hives have symptoms for longer than 1 year, 50% have symptoms for longer than 5 years, and 20% have symptoms for decades. In 50% of the cases, the offending agent is not identified.
Diagnosing chronic hives is more involved than diagnosing acute hives. A very detailed history is taken to determine the exact appearance and duration of the hives. A food diary should be kept, and all medications documented. The time course of the hives should also be covered. For example, do the symptoms persist when not at work or while on vacation at a different location? Some chronic hives are caused by infections that were not realized, especially sinus infections. Chronic hives are also associated with thyroid disease - hypothyroidism and hyperthyroidism. An infection with the yeast, Candida albicans, is also associated with chronic hives. All of these conditions should be ruled out. Skin testing is of variable benefit since people with hives tend to react positively to most agents tested for.