15.3 Direct Detection Methods and HPV Cervical Screening

15.3 Direct Detection Methods and HPV Cervical Screening

Quick Answer: Direct detection means finding viral antigen, viral nucleic acid, or (less often) infectious virus in the specimen—not host antibody. Contemporary M practice is NAAT first: multiplex respiratory panels and meningitis/encephalitis panels. Antigen EIAs for influenza and RSV are faster but miss infection. DFA still appears in older protocols. Viral culture is largely historical except CMV shell vial. Cervical cancer screening uses high-risk HPV NAAT with cytology. Collect nasopharyngeal swabs in UTM, vesicle swabs in VTM, stool for GI viruses, plasma for viral load, and CSF for meningitis. Antiviral resistance mechanisms are SM-only.

Serology detects the host response. Direct detection detects the virus. The M exam will give you a syndrome, a specimen, and a method and expect you to know which combination is valid today.

Antigen detection and DFA

Enzyme immunoassays and lateral-flow antigen tests for influenza and RSV were the original rapid respiratory tests. They detect viral protein in nasopharyngeal secretions and can return a result in minutes. Compared with NAAT they are faster but less sensitive, especially when viral load is falling, when the swab is poorly collected, or when the circulating influenza strain is a mismatch for the capture antibodies. A positive antigen result in the right season can support treatment decisions; a negative antigen result does not rule out disease when clinical suspicion is high and should reflex to NAAT. SARS-CoV-2 antigen tests follow the same sensitivity caveat. Antigen EIAs also exist for rotavirus and some norovirus assays on stool, again with lower sensitivity than molecular methods.

Direct fluorescent antibody (DFA) staining applies fluorescein-labeled monoclonal antibodies to cells from a nasopharyngeal wash, a vesicle scraping, or a centrifuged shell-vial monolayer. Bright intracellular fluorescence indicates infected cells. DFA has been used for RSV, influenza, HSV, and VZV. It is more specific than older cytologic stains such as Tzanck, which cannot distinguish HSV from VZV, but DFA still requires adequate cellular material and a fluorescent microscope. Many laboratories have retired respiratory DFA in favor of multiplex PCR. Know DFA as antigen detection in cells, not as a nucleic acid method and not as serology.

NAAT and multiplex panels

Nucleic acid amplification—PCR, RT-PCR, and related chemistries—is the reference direct-detection method for most M-tested viruses. It is more sensitive than antigen testing, does not require viable virus, and can multiplex many targets from one swab or aliquot of CSF.

Respiratory multiplex NAAT typically includes influenza A and B, RSV, SARS-CoV-2, parainfluenza, adenovirus, human metapneumovirus, and rhinovirus/enterovirus. A nasopharyngeal swab in UTM is the usual source. The panel does not replace clinical correlation: a detected rhinovirus in an intubated adult may be carriage, whereas influenza A in a febrile patient is disease. Inhibitors, inadequate swabbing, and testing too late in illness still produce false negatives.

Meningitis/encephalitis multiplex NAAT on CSF commonly includes HSV-1, HSV-2, VZV, enterovirus, CMV, and HHV-6 (plus bacterial and cryptococcal targets on some cartridges). CSF is the matched matrix; a vesicle swab cannot diagnose HSV meningitis. HSV PCR on CSF is time-critical because HSV encephalitis is treatable; do not wait for culture. Enterovirus PCR on CSF explains most community aseptic meningitis in season.

Quantitative NAAT (viral load) on plasma is used for HIV, CMV, HBV DNA, HCV RNA, and BK virus. Reportable units are copies/mL or IU/mL. Trends on the same assay matter more than a single number from a different platform. Whole blood, serum, and plasma are not automatically interchangeable; the assay’s validated matrix is required. Stool NAAT is the direct method of choice for norovirus when an outbreak or severe illness must be documented.

Viral culture: limited contemporary use

Conventional tube culture inoculates specimen onto cell lines (for example MRC-5, RMK, A549) and waits days to weeks for cytopathic effect, followed by DFA identification. Many clinically important viruses will not grow in routine cell culture: norovirus, HPV, hepatitis C, and SARS-CoV-2 in standard clinical setups. Turnaround is too slow for influenza and HSV management. Culture also requires viable virus, so a swab in the wrong medium or delayed transport kills the test.

The method that remains in teaching and in some transplant laboratories is CMV shell-vial culture. Specimen (urine, respiratory, or leukocyte-rich blood, depending on protocol) is centrifuged onto a cell monolayer to enhance adsorption, incubated briefly, and stained with monoclonal antibody to CMV immediate-early antigen. The centrifugation-plus-early-antigen readout is faster than waiting for cytopathic effect. HSV shell vials exist historically but have been displaced by lesion PCR. For the exam: culture is not the first-line method for respiratory panels, GI viruses, HIV, or HPV; CMV shell vial is the culture format still worth naming.

HPV cervical cancer screening

High-risk HPV types, especially 16 and 18, cause the majority of cervical cancers. Screening is a laboratory partnership between high-risk HPV NAAT and cervical cytology (Pap). Depending on age and guidelines, HPV NAAT may be used as primary screening, as co-testing with cytology, or as a reflex after an atypical Pap result. Genotyping that separately reports HPV 16/18 identifies people who need colposcopy even when cytology is negative. This is direct detection of viral nucleic acid in a cervical specimen, not serology and not culture—HPV is not grown in clinical cell culture. Low-risk HPV types that cause genital warts are not the cervical-screening target. Liquid-based cytology vials are validated collection devices for both the slide and the HPV assay; a random dry swab is not a substitute unless the assay manufacturer says so.

Specimens and transport

Labeling, collection time, and transport temperature are preanalytic rules from earlier chapters, but they decide whether a virus assay is valid. Calcium alginate and some wooden-shaft swabs inhibit PCR. Bacterial transport gel is not viral transport medium. Stool in polyvinyl alcohol or formalin is a parasite fixative, not a virus specimen.

What M does not test

Do not memorize neuraminidase-inhibitor binding sites, CMV UL97 kinase mutations, HIV reverse-transcriptase resistance genotypes, or HBV polymerase mutations. Antiviral mechanisms and resistance are SM-only. M asks you which method detects the virus in which specimen, and how HPV screening is performed with cytology—not how a drug binds a viral enzyme.