Curr Infect Dis Rep (2012) 14:275–283 pandemic influenza was found to be underestimated by com- monly used pneumonia severity scores [22 , 23]. Avian influ- enza (H5N1) frequently presents as severe primary pneumonia that often progresses rapidly to the acute respiratory distress syndrome (ARDS), having caused high rates of death, espe- cially among infants and young children in Southeast Asian countries [24]. Laboratory Testing In certain situations, confirmation of etiology by laboratory testing is required in order to guide the initiation and duration of antiviral therapy, and for the implementation of infection control measures and surveillance. Other benefits of influenza virus detection are the reduction of inappropriate antibiotic use, decreased length of stay in emergency departments, and fewer additional laboratory studies, all leading to a reduction in health care costs [1 ]. The Centers for Disease Control and Prevention (CDC) and the Infectious Diseases Society of America (IDSA) have published guidelines to better define patients who should undergo influenza testing [1 , 2 ]. The available methods include immunological techniques (i.e. rapid antigen-based tests, immunofluorescence assays, sero- logic testing), molecular techniques (i.e. reverse-transcriptase polymerase chain reaction [RT-PCR]), and microbiological techniques (i.e. viral cultures). While RT-PCR has the highest sensitivity and specificity, rapid antigen and immunofluores- cence testing, though very useful as initial screening tests, are considerably less sensitive. Besides, the sensitivity and spec- ificity of any of these techniques will vary depending on the laboratory equipment, personnel expertise, the timing of rec- ollection, and the appropriate handling of the samples [1 , 2 ]. Respiratory specimens can be obtained by many different methods including throat swabs, nasal aspirates, and nasopha- ryngeal swabs, aspirates and washing [2 ]. In mechanically ventilated subjects, more invasive maneuvers such as endo- tracheal aspirates and bronchoscopic or non-bronchoscopic bronchoalveolar lavage (BAL) may be required in order to obtain adequate lower respiratory tract samples. While rapid antigen tests, immunofluorescence and RT-PCR all can yield results quick enough to guide point-of-care clinical decision- making, serologic tests and viral cultures provide retrospective diagnosis, availability only in reference laboratories, and use- fulness when confirming screening test. For this reason, they are normally reserved for epidemiological and research pur- poses (Table 2). Rapid Influenza Diagnostic Tests Rapid influenza diagnostic test (RIDT) are designed to detect influenza A and B nucleoproteic antigens in respiratory 277 specimens, with results expressed qualitatively as positive or negative in no more than 15 min. Many different FDA- approved tests are available; while some assays are capable of distinguishing between influenza A or B viruses, none can distinguish between pandemic and seasonal strains of influenza A [1 , 2 ]. Compared to reference methods (i.e. RT-PCR and viral culture), they have low sensibility (10 % to 80 %) and high specificity (95 % to 100 %), and there is also great variability between the different commercially available kits [25–27]. Sensitivity appears to be somewhat lower for influen- za B than that seen for influenza A [26]. Also, the reported sensitivity for rapid tests is higher for nasopharyngeal samples than for throat swabs [27]. There are two main factors that influence RIDT’s negative predictive value. First, negative results obtained during periods of high viral activity in the community are more likely to be false negatives; alternatively, false positives, though much less frequent, are more likely during periods of less viral circulation. Second, respiratory samples collected within the first 48 to 72 h of symptom onset positively influence RIDT’s sensitivity. For these reasons, when rapid tests are negative, confirmation by means of RT- PCR or viral culture should be considered [1 , 2]. Compared to RT-PCR, the sensitivity of RIDTs for detecting novel influ- enza A (H1N1) was equal to or lower than the sensitivity to detect seasonal influenza viruses [29–32], so RIDT results need to be interpreted with caution when evaluating patients sus- pected of having pandemic H1N1 influenza A. Immunofluorescence Direct (DFA) or indirect (IFA) immunofluorescence anti- body staining techniques are capable of detecting influenza A and B viruses, and distinguish the viruses from each other as well as from other respiratory viruses [1 , 2 ]. They have levels of sensitivity and specificity that come close to those of RT-PCR [28, 33], and results are often available in a few hours [1 , 2 ]. Although these tests have improved sensitivity over RIDT, they are more technically complex and require exper- tise in obtaining quality respiratory samples (Table 2). Reverse-Transcriptase Polymerase Chain Reaction This is the reference influenza detection method and has the highest sensitivity and specificity [34, 35]. Several modali- ties of RT-PCR have been designed: conventional gel-based PCR (cRT-PCR), multiplex PCR (mRT-PCR), and real-time RT-PCR (rRT-PCR). They can differentiate between influ- enza types (A or B) and subtypes (including pandemic H1N1 influenza and avian H5N1 influenza), and results are available in 2–6 h (although due to transportation of batched specimens to reference centers for processing, it may take longer for