Indications for Testing
Testing to diagnose hMPV should be considered, especially during winter and spring (CDC), in the following groups of people who present with respiratory illness :
- Infants and children <10 years with severe respiratory illness
- Adults >65 years
- Immunocompromised patients (children and adults with malignancy or hematopoietic stem cell transplants)
- Patients with exacerbated chronic medical conditions (chronic obstructive pulmonary disease [COPD] or asthma) or comorbidity (lung transplant)
- Preterm infants or those with congenital heart disease, whose conditions increase their risk for severe pneumonia
- Individuals with unexplained respiratory illness in long-term care facilities
In addition to testing for hMPV with direct fluorescent antibody (DFA) staining or a polymerase chain reaction (PCR) assay, concurrent viral testing for other respiratory infections, especially respiratory syncytial virus (RSV) or adenovirus, or for atypical pneumonia agents can be considered, as coinfections do occur. Collection of a high-quality, cellular respiratory specimen is essential for best results.
Direct Fluorescent Antibody Testing
DFA staining is ideal as an initial test for both adults and children because it allows for simple, rapid detection of viral proteins from clinical samples at low cost and with acceptable sensitivity, thereby enabling early medical management. The sensitivity is highly dependent on an adequate specimen and the point during the illness at which the specimen is collected. When it is used, DFA should be considered a first-line approach; PCR testing should then be added to confirm negative results.
Polymerase Chain Reaction Testing
Molecular testing by reverse-transcriptase PCR (RT-PCR) in nasopharyngeal swabs and aspirates is highly sensitive in diagnosing hMPV, as well as other respiratory viruses. RT-PCR assays for hMPV as a single target or as part of a viral respiratory panel are available and offer higher sensitivities than DFA staining, but these tests are more expensive and may have slower turnaround times, which can impact patient care. Because of its higher sensitivity, PCR testing should be used to confirm a negative DFA result, especially in children and immunocompromised patients in whom hMPV is still suspected.
Viral culture is not recommended because hMPV is difficult to isolate in conventionally used cell lines and grows slowly and inefficiently in cell culture, requiring >2 weeks before the onset of damage in host cells; it also has low sensitivity.