DIAGNOSIS H1N1

Guidelines for the diagnosis of swine H1N1 influenza A virus have been released by the United States Centers for Disease Control and Prevention (CDC) . Updated recommendations can be found at the CDC’s website (http://www.cdc.gov/swineflu/). Clinicians in other countries should consult their individual health ministries for information about recommended diagnostic testing.

Specimens — When swine H1N1 influenza A is suspected, an upper respiratory sample (nasopharyngeal swab/aspirate or nasal wash/aspirate) should be collected as soon as possible . If these samples are not able to be obtained, a combined nasal swab with an oropharyngeal swab should be collected. In intubated patients, an endotracheal aspirate should also be obtained.

Swabs with a synthetic tip (eg, polyester or Dacron) and an aluminum or plastic shaft should be used. Swabs with cotton tips and wooden shafts are not recommended. Swabs made of calcium alginate are not acceptable. The collection vial in which the swab is placed should contain 3 mL of viral transport media.

Specimens should be placed in viral transport media and placed on ice (4ºC) or refrigerated immediately for transportation to the laboratory. Once the samples arrive in the laboratory, they should be stored either in a refrigerator at 4ºC or in a -70ºC freezer. If a -70ºC freezer is not available, they should be kept refrigerated, preferably for ≤1 week.

Specimens should be shipped on dry ice to the state public health laboratory in clearly labeled containers and should include all information requested by the state health laboratory.

State laboratories in the United States should will send all unsubtypable influenza A specimens to the CDC for further testing.

Recommended tests — In the United States, the recommended test for suspected cases of swine H1N1 influenza A virus is real-time reverse transcriptase (RT)-PCR for influenza A, B, H1, and H3 . The strain of swine H1N1 influenza A virus associated with the 2009 outbreak tests positive for influenza A and negative for H1 and H3 by real-time RT-PCR. RT-PCR testing is generally performed at a state health department laboratory; confirmatory testing is performed at the CDC. Case definitions of confirmed, probable, and suspected swine H1N1 influenza A virus infection are discussed below. (See “Case definitions” belowSee “Case definitions” below).

Other tests — Certain rapid influenza antigen tests that are commercially available can distinguish between influenza A and B viruses [19]. Thus, a patient with only influenza B virus infection would not be suspected of havingswine H1N1 influenza A virus infection. In contrast, a patient with a positive rapid antigen test for influenza A may be considered a probable case if he or she meets the other criteria (see “Case definitions” belowsee “Case definitions” below). However, the sensitivity and specificity of rapid antigen testing for swine H1N1 influenza A virus infection is unknown and poor sensitivity has already been demonstrated for seasonal influenza. A negative rapid influenza test does not exclude infection.

Direct of indirect immunofluorescent antibody testing (DFA or IFA) can distinguish between influenza A and B. Thus, a patient with a positive DFA or IFA may be considered a probable case if he or she meets the other criteria (see “Case definitions” belowsee “Case definitions” below). A negative DFA or IFA does not exclude swine H1N1 influenza A infection since these tests have unclear sensitivity to detect this virus.

Isolation of swine H1N1 influenza A virus using culture is diagnostic, but culture is usually too slow to help guide clinical management. A negative viral culture does not exclude swine H1N1 influenza A infection.

The diagnostic tests for influenza are discussed in greater detail separately. (See “Clinical manifestations and diagnosis of influenza in adults” and see “Clinical features and diagnosis of influenza in children”)

CASE DEFINITIONS — Definitions are changing as we learn more about this virus and the syndromes it causes. Updated definitions can be found at the CDC’s website (http://www.cdc.gov/swineflu/). Cases in the United States are confirmed by diagnostic testing at the Centers for Disease Control and Prevention [1,19,27]. (See “Diagnosis” aboveSee “Diagnosis” above).

The following current case definitions have been provided by the United States Centers for Disease and Prevention and are current as of April 29, 2009 :

  • A confirmed case of swine H1N1 influenza A is defined as an individual with an acute febrile respiratory illness (a measured temperature of 37.8ºC [100.4ºF] and recent onset of at least one of the following: rhinorrhea or nasal congestion, sore throat, or cough) with laboratory-confirmed swine H1N1 influenza A virus detection by real-time reverse transcriptase (RT)-PCR or culture.
  • A probable case of swine H1N1 influenza A is defined as an individual with an acute febrile respiratory illness who is positive for influenza A, but negative for H1 and H3 by RT-PCR
  • A suspected case of swine H1N1 influenza A is defined as an individual with an acute febrile respiratory illness who:

      – Develops symptoms within seven days of close contact with a person who is a confirmed case of swine H1N1 influenza A virus infection or

      – Develops symptoms within seven days of travel to a community either within the United States or internationally where there are one or more confirmed swine H1N1 influenza A cases or

      – Resides in a community where there are one or more confirmed swine H1N1 influenza A cases (See “Epidemiology” above).

REFERENCE

  1. United States Centers for Disease Control and Prevention. Interim guidelines on antiviral recommendations for patients with confirmed or suspected swine influenza A (H1N1) virus infection and close contacts. http://www.cdc.gov/swineflu (Accessed April 29, 2009).
  2. United States Centers for Disease Control and Prevention. Interim Guidance for Clinicians on Identifying and Caring for Patients with Swine-origin Influenza A (H1N1) Virus Infection http://www.cdc.gov/swineflu/identifyingpatients.htm (Accessed April 29, 2009).
  3. United States Centers for Disease Control and Prevention. Interim Guidance on Case Definitions to be Used For Investigations of Swine Influenza A (H1N1) Cases. http://www.cdc.gov/swineflu/casedef_swineflu.htm (Accessed April 27, 2009).
  4. United States Centers for Disease Control and Prevention. Interim Guidance for Clinicians on the Prevention and Treatment of Swine-Origin Influenza Virus Infection in Young Children. http://www.cdc.gov/swineflu/childrentreatment.htm (Accessed April 29, 2009).
  5. United States Centers for Disease Control and Prevention. Interim Guidance — Pregnant Women and Swine Influenza: Considerations for Clinicians. http://www.cdc.gov/swineflu/clinician_pregnant.htm (Accessed April 29, 2009).

 

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