Diagnosis

Differentiating between a cold and flu may be difficult. Cold symptoms are nearly always less severe than those of the flu.

Comparing Colds and Flus

Symptoms	Cold	Flu
Fever	Rare	Common and high (102-104° F); lasts 3 - 4 days
Headache	Rare	Almost always present
General aches and pains	Mild, if they occur at all	Often severe
Fatigue, exhaustion, and weakness	Mild, it they occur at all	Extreme exhaustion is early and severe; can last 2 - 3 weeks
Stuffy nose	Nearly always	Sometimes
Sneezing	Very common	Sometimes
Sore throat	Common	Sometimes
Chest discomfort and cough	Mild to moderate, hacking cough	Common, can be severe
Source: National Institute of Allergy and Infectious Disease

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Diagnosing the Flu

Several available tests can isolate and identify the viruses responsible for some respiratory infections. They are generally not needed, since most cases of the flu are self-evident. However, such tests can be very helpful in confirming or ruling out the flu. If a doctor believes a diagnosis would help, samples using a swab should be taken from the nasal passages or throat within 4 days of the first symptoms.

A nasopharyngeal culture is a test used to identify disease-causing organisms in nasal secretions. Nasopharyngeal cultures are useful in identifying Bordetella pertussis and Neisseria meningitidis (types of bacteria). The culture may help determine appropriate antibiotic therapy.

Several rapid tests for the flu can produce results in less than 30 minutes, but vary on the specific strain or strains that they can detect. They are not as accurate as a viral culture, however, in which the virus is reproduced in the laboratory. Culture results can take 3 - 10 days. Blood tests can also document the infection several weeks after symptoms appear.

Diagnosing Avian Influenza

In February 2006, the U.S. Food and Drug Administration approved a new, faster test for diagnosing strains of avian influenza (bird flu) in people suspected of having the virus. The test is called the Influenza A/H5 (Asian lineage) Virus Real-time RT-PCR Primer and Probe Set. The test gives preliminary results within 4 hours. Older tests required 2 - 3 days.

Ruling out Other Causes of Congestion

Ruling out Allergic Rhinitis. Symptoms of allergic rhinitis include nasal obstruction and congestion, which are similar to the symptoms of a cold. People with allergies, however, are likely to have the following: