Diagnostic Virology — Questions

a. The result is uninterpretable, not a true negative. The internal control is there to prove the reaction could amplify; because it failed, amplification was blocked, so a negative viral target cannot be trusted. That the positive and no-template controls behaved normally localises the problem to this sample, not the run.

b. The likely cause is inhibition by something in the specimen, commonly blood, heparin, excess salts or reagents carried over from extraction, or an extraction failure or a lost or mishandled sample.

c. The sample must be re-processed, not reported as negative: re-extract and repeat, and if inhibition persists dilute the sample to reduce the inhibitor and repeat. If it still fails, report the result as inhibited and request a fresh specimen rather than issue a false negative.

An indeterminate result means the serum reacts with fewer than the full panel of HCV antigens the immunoblot needs to call a definite positive. The historical recombinant immunoblot assay required at least two of four bands (core, NS3, NS4, NS5); a single band was reported as indeterminate.

Three explanations exist:

• Resolved past infection with waned antibody breadth. HCV RNA is negative.
• Early or acute infection with a maturing response. HCV RNA may be positive.
• False-positive cross-reactivity to one antigen. HCV RNA is negative and the screen should be repeated on another platform.

HCV RNA polymerase chain reaction (PCR) is the decisive test: a positive result confirms active infection whatever the immunoblot shows, and a negative result excludes it. Modern algorithms have largely dropped the immunoblot in favour of direct RNA confirmation of any reactive screen.

Both are functional assays, detecting what antibody does rather than that it binds, and both are read as titres.

• Haemagglutination inhibition detects antibody by its ability to block virus-induced agglutination of red cells. It is quick and needs no cell culture, used for agglutinating viruses such as influenza.
• Plaque (neutralisation) assay measures antibody that blocks viral infectivity in cultured cells, counting the reduction in plaques. It correlates with protection and is a reference standard, but it is slow, labour-intensive and uses live virus.

Both have been largely replaced by enzyme immunoassay for routine work, surviving mainly in reference and vaccine-evaluation settings.

A false-negative CSF PCR arises before, during or after the assay.

• Timing. The specimen may be taken too early, before the target is detectable, or too late, after the virus has cleared from the cerebrospinal fluid.
• Low target. CSF volumes are small and viral loads in the central nervous system can be low, so the target may fall below the limit of detection.
• Inhibition. Substances in the sample, particularly blood from a traumatic tap, inhibit the polymerase; an internal control that fails alongside a negative target flags this rather than a true negative.
• Assay factors. Primer-site sequence variation in a divergent strain, RNA degradation, or extraction failure can each abolish amplification.

• Principle. Antibody made early in infection binds weakly; after affinity maturation in germinal centres it binds strongly, so IgG avidity separates recent from past primary infection.
• Affinity versus avidity. Affinity is the strength of a single antibody–antigen bond; avidity is the total binding of a complex, set by the individual affinities and the valency.
• Method. Two parallel ELISAs on the same sample, one with a brief chaotropic agent step (urea, diethylamine, ammonium thiocyanate) after binding that strips low-avidity bonds but not high-avidity ones.
• Avidity index. AI = (OD with chaotrope) ÷ (OD without chaotrope) × 100. Low (below ~30 to 40%) indicates recent primary infection in the past two to four months; high (above ~60%) indicates infection at least three to six months earlier.
• Clinical use. Greatest value is in pregnancy screening, where the timing of maternal infection sets fetal risk: high early-pregnancy avidity for toxoplasma, cytomegalovirus, or rubella excludes recent primary infection, while low avidity raises concern. It also clarifies persistent or non-specific IgM by checking whether IgG has matured.

The TaqMan probe binds a sequence within the amplicon, between the two primers, and carries a reporter dye at one end and a quencher at the other. While the probe is intact the quencher sits close enough to the reporter to suppress its fluorescence.

During the extension step the 5’ exonuclease activity of Taq polymerase cleaves the bound probe, releasing the reporter dye away from the quencher so that it emits light on excitation. Because a probe is hydrolysed for each new amplicon, the fluorescent signal is proportional to the product made, and detection is sequence-specific because signal arises only when the probe has hybridised to its target.

• Raise the annealing temperature. A higher temperature allows only closely matched primer-template duplexes to remain bound, favouring specific over non-specific priming.
• Lower the salt (magnesium) concentration. Less magnesium destabilises mismatched binding, so the reaction tolerates less mismatch. Both changes trade some sensitivity for greater specificity.

• Gel electrophoresis. The amplicon is separated by size and visualised with an intercalating dye such as ethidium bromide that fluoresces under ultraviolet light.
• Real-time fluorescence. In real-time PCR the product is detected as it accumulates through a fluorescent dye or sequence-specific probe, with no separate post-amplification step.

• Capture step. The plate is coated with anti-human IgM antibody, which captures all IgM from the patient’s serum (specific and non-specific); other classes (IgG, IgA) are washed away.
• Detection step. Viral antigen is added and binds only the IgM of matching specificity; bound antigen is then detected with a labelled anti-viral reagent, giving a signal proportional to virus-specific IgM.
• Why the capture format is preferred. It solves two problems of a direct IgM ELISA: rheumatoid factor (itself an anti-IgG IgM) cannot cause false positives because no IgG remains in the captured fraction, and high-titre virus-specific IgG cannot outcompete IgM for antigen because it has been washed off.
• Clinical use. The test of choice for recent or current infection, since IgM appears in the first one to two weeks and decays over months. Applications include cytomegalovirus, rubella, toxoplasma, and parvovirus B19 in pregnancy, newborn screening for intrauterine infection (IgM does not cross the placenta), and anti-HBc IgM in acute hepatitis B. Combining it with IgG avidity improves discrimination of persistent IgM.

CRISPR-based detection uses a programmable nuclease guided to a specific viral sequence, adding a layer of sequence recognition beyond the primers of an amplification reaction.

In practice it is coupled to an isothermal amplification step such as RT-LAMP: once the target is amplified and recognised by the guide, the activated nuclease shows collateral cleavage of a labelled reporter, generating a fluorescent or lateral-flow signal. The attractions for diagnostics are a rapid, low-cost, point-of-care format needing minimal instrumentation, and, because many CRISPR nucleases are available, the possibility of highly parallel genotyping in a single reaction. It confirms that the intended species was amplified, improving specificity over amplification alone.

A complete answer defines the two performance measures, shows why sensitivity alone is insufficient, and links this to how serological testing is structured.

Sensitivity is the proportion of truly infected people a test calls positive, and a screening assay is deliberately built for it so that few true cases are missed. But sensitivity says nothing about the false positives a test produces. Specificity, the proportion of truly uninfected people called negative, governs that, and a test can be very sensitive yet poorly specific.

The practical consequence is that predictive value depends on prevalence. In a low-prevalence setting, even a specific test yields a low positive predictive value, because most positives are false. A single sensitive result is therefore not a diagnosis.

This is why serology is built as screen-then-confirm: a sensitive first assay to avoid missing infection, then a specific confirmatory assay to remove false positives, with every result read against the patient’s pre-test probability.

A complete answer weighs the clinical gains against the interpretive and technical costs, then gives the practical ways a laboratory manages them.

Advantages. A single respiratory, meningitis-encephalitis or gastrointestinal cartridge tests one specimen for a dozen or more causes in about an hour. This gives broad, fast coverage when the differential is wide, supports empirical treatment and isolation decisions that cannot wait for sequential testing, and needs little hands-on time.

Challenges. Packing many reactions into one tube risks competition between targets and reduced sensitivity for any single one against a dedicated assay. Broad panels detect organisms of uncertain clinical significance, including prolonged shedding or incidental carriage, so a positive is not automatically the cause of the presentation. Cost per test is high, and a long result list can mislead if read uncritically.

Solutions. Match the panel to the clinical question rather than testing everything reflexively, confirm or quantify a key positive with a targeted assay where it changes management, and report with interpretative comment so the clinician reads each result against the clinical picture.

A complete answer tracks each generation by its antigen, what it detects, and how each step narrowed the window between infection and a positive result.

First-generation assays coated the plate with whole viral lysate and detected IgG only, so they were relatively insensitive early and prone to cross-reactivity.

Second-generation assays replaced lysate with recombinant proteins or synthetic peptides, improving specificity and sensitivity.

Third-generation assays used an antigen-sandwich (double-antigen) format that detects IgM as well as IgG, becoming positive earlier and so shortening the window.

Fourth-generation assays are combined tests that add p24 antigen detection to antibody detection. Because p24 appears before antibody, this shortens the window further, narrowing the interval in which a recently infected, infectious person tests negative.

A complete answer groups the chemistries by what they multiply, target, signal or probe, then names the main examples of each with a line on how it works.

Target amplification makes more of the target sequence. PCR is the prototype, using flanking primers and a thermostable polymerase through cycles of denaturation, annealing and extension; an RT step (RT-PCR) is added for RNA. Variants include nested PCR (two primer rounds for sensitivity), multiplex PCR (several primer sets, several targets), and the isothermal methods that need no thermal cycler: NASBA and TMA (transcription-based, for RNA), SDA (strand displacement), and LAMP (loop-mediated, well suited to point-of-care).

Signal amplification leaves the target untouched and multiplies the detection signal. The branched DNA (bDNA) assay builds a tree of enzyme-labelled probes onto the captured target; hybrid capture detects RNA-DNA hybrids with an antibody and a chemiluminescent readout. Both avoid amplicon carryover and give stable quantification.

Probe amplification multiplies a probe rather than the target, the ligase chain reaction (LCR) joining adjacent probes only on a matching target.

Real-time detection chemistries sit alongside these: SYBR Green binds any double-stranded DNA and needs melting-curve analysis for specificity, while sequence-specific hydrolysis (TaqMan), hybridisation (FRET) and molecular-beacon probes confirm the amplicon’s sequence directly.

A complete answer covers the analytical, bioinformatic and interpretive problems that make an NGS resistance assay harder to validate than a single-target PCR.

The central analytical problem is the minority variant: NGS can report mutations present in a small fraction of the population, so validation must set a detection threshold and prove it separates true low-frequency variants from PCR and sequencing error. This means establishing a limit of detection and reproducibility across runs and controlling error rates, since a threshold set too low reports noise and one set too high misses real resistance.

The bioinformatic pipeline is part of the assay and must itself be validated and version-controlled, because a change in software or reference can change the result. Performance is anchored with reference standards, external quality assessment and concordance against Sanger sequencing as the established comparator.

Interpretation is unsettled: the clinical significance of small minority resistant populations is still uncertain, and sensitivity depends on the input viral load, so a low-load sample may not yield reliable variant calling.

Both qualitative and quantitative nucleic-acid tests amplify HIV nucleic acid by reverse-transcription PCR (or transcription-mediated amplification, TMA), but they answer different clinical questions.

Comparison

Similarities

Both assays:

• Use reverse-transcription PCR or TMA to detect HIV nucleic acid.
• Are subject to PCR inhibition (heparin, haemoglobin, urine, faecal contamination, etc.); internal controls flag failed runs.
• Are vulnerable to subtype-specific primer mismatches if not properly validated for the local subtype (subtype C in South Africa).
• Detect virus earlier than serology, closing the diagnostic window.
• Require careful specimen handling to preserve RNA integrity (timely separation of plasma; appropriate shipping).

Key differences

• Purpose. Qualitative PCR answers “infected or not”; quantitative viral load (VL) answers “how well is therapy working”.
• Quantification. Qualitative outputs no number; quantitative outputs a precise viral concentration that drives clinical action (repeat-and-persistent-check eligibility at below 50 copies/mL; low-level viraemia between 50 and 999 copies/mL; failure assessment at 1,000 copies/mL or above).
• Indeterminate handling. Qualitative PCR has a defined indeterminate category and algorithm; quantitative VL produces a clean numerical result.
• Confirmation requirement. A positive qualitative result in a child under 2 always requires confirmation; a quantitative VL is a clinical trajectory, not a single-result diagnosis.

Clinical implications

• Early infant diagnosis uses qualitative PCR because the question is binary (infected or not) and the test must operate near the diagnostic threshold.
• ART monitoring uses quantitative VL because the clinically meaningful information is the numerical trajectory (rising, falling, stable around suppression).
• DBS is acceptable for both purposes in surveillance contexts but has greater quantitative inaccuracy.
• A qualitative result is not a substitute for a viral load in an established patient on ART, and vice versa for an infant diagnosis.

The discordance is unidirectional (new positive, old negative), so the answer weighs the competing explanations and then sets out a staged resolution.

Possible causes

True positives the old assay missed. The new platform may be more sensitive (a third-generation design detecting core, NS3, NS4 and NS5), or the samples were taken in the window period with antibody below the old cut-off. This is the most clinically important possibility, because the new assay is right.

False positives from the new assay. Lower specificity at the current cut-off, non-specific binding in autoimmune disease, rheumatoid factor or heterophile antibody cross-reactivity, or miscalibration.

Sample-related. Storage degradation at minus 20 (rather than minus 70), freeze-thaw effects, contamination or plate carry-over from high-positive wells, or mislabelling.

Reagent and operational. Lot variability, calibrator drift, plate-edge effects, operator error. Rarely, an original false-negative on the day.

Investigation

• Repeat the 10 samples in duplicate on the new platform with known controls, and inspect QC charts for run excursions.
• Cross-platform comparison on at least one alternative anti-HCV assay to triangulate the true status.
• HCV RNA nucleic acid testing as the decisive marker: a positive resolves the discrepancy in favour of the new assay, while a negative does not exclude resolved past infection. HCV core antigen is a secondary virological marker.
• Clinical and risk-factor review of the 10 patients, and a fresh sample where possible to exclude storage artefact.
• Recompute sensitivity and specificity against a corrected reference (RNA plus cross-platform agreement).

Conclusion

If most discordants are RNA-confirmed, the new assay has improved sensitivity and should be adopted with reflex RNA confirmation. If most are RNA-negative and negative elsewhere, it has a specificity problem needing recalibration or rejection. The 100% concordance on the 50 known positives confirms acceptable sensitivity, so the decision rests on specificity.

The patient’s antibody is sandwiched between antigen on both sides, so the format detects virus-specific antibody, not antigen. Needing specific binding on both arms also makes it more specific than an indirect assay.

It is not an antigen assay here, and it will not register rheumatoid factor, non-specific immunoglobulin or complement, because only a specific antibody can bridge the two antigens.

A fourfold or greater rise in IgG titre across paired sera shows an active, evolving antibody response and so marks recent infection. No change would indicate past exposure.

A stable titre is past immunity or carriage, a genuine rise is not an error, and passive antibody would decline rather than rise.

p24 antigen appears before antibody, so a combined assay that detects it alongside antibody turns positive earlier and narrows the window. It remains a serological assay, not a molecular one.

Fourth-generation assays do not detect viral RNA (that is a nucleic acid test), and neutralising antibody, avidity and rheumatoid factor play no part in shortening the window.

Each cycle doubles the target, so a three-cycle-earlier threshold means about 2^3, or eightfold, more starting template. The cycle threshold falls as input rises.

Three cycles is not a threefold difference (a linear misreading), and 30 or 300 fold would correspond to far larger cycle gaps.

Antibody is absent for some weeks after infection, so a sample taken this early falls in the diagnostic window and reads negative even though infection is present. A convalescent sample is needed.

The virus does induce antibody in time; rheumatoid factor and the hook effect cause false results in other settings; and passive maternal antibody is irrelevant in an adult.

A mismatch at the 3’ end is poorly tolerated because that is where the polymerase begins extension, so a mismatched 3’ base blocks efficient synthesis. Mismatches nearer the 5’ end are better tolerated.

Extension begins at the 3’ end, not the 5’ (excluding A and E), and tolerance is not uniform along the primer (excluding B and C).

Clean DNA reads about 1.8; a lower ratio points to protein or phenol contamination, which absorb near 280 nm and depress the ratio. Such contaminants can also inhibit downstream PCR.

Pure DNA would sit near 1.8, RNA contamination would raise not lower the ratio, and the value reflects contamination rather than magnesium or an empty eluate.

At an extremely high analyte level the capture and detector reagents are saturated separately, so few complete sandwiches form and the result reads falsely low or negative: the high-dose hook effect. Diluting the sample restores a positive.

The window and passive antibody concern antibody assays, rheumatoid factor causes false positives, and an assay defeated by excess antigen is not one of low sensitivity.

Taq polymerase copies only DNA, so an RNA target must first be reverse-transcribed into complementary DNA (RT-PCR) before amplification.

Ligation, restriction digestion and alkaline denaturation are not part of RT-PCR, and uracil-N-glycosylase is a contamination-control measure, not a route to copying RNA.

Magnesium is the polymerase cofactor, and in excess it stabilises mismatched primer-template binding and undenatured double-stranded DNA, lowering specificity (and yield). Its concentration is therefore optimised carefully.

Too little magnesium starves the enzyme, but it does not degrade the polymerase, block reverse transcription, or govern the denaturation temperature.

The difference is the label: ECLIA uses a ruthenium label that emits light when triggered electrically at an electrode, rather than the enzyme-and-substrate colour reaction of an ELISA. The immunoassay format is otherwise unchanged.

Both can detect antigen or antibody, both use a solid phase, and ECLIA is in fact the basis of fully automated high-throughput analysers, not a functional cell-culture assay.

In the indirect format, patient antibody binds the fixed antigen, and the enzyme-labelled anti-human immunoglobulin then binds that patient antibody, generating signal.

The detector does not bind the antigen or free antigen directly, there is no capture antibody in an indirect assay, and it binds human immunoglobulin generally, not only rheumatoid factor.

A dideoxynucleotide lacks the 3’ hydroxyl group needed to form the next phosphodiester bond, so extension stops once it is incorporated. The labelled terminator marks the final base of each fragment.

The dideoxynucleotide keeps its 5’ phosphate and base, carries a fluorescent label, and chain termination is a covalent-extension problem, not a hydrogen-bonding one.

Antibody avidity matures with time, so low-avidity IgG marks recent primary infection while high-avidity IgG marks past infection. This is why avidity testing helps date an infection, notably in pregnancy.

Past infection gives high avidity, passive antibody reflects the donor’s mature response, and avidity is a property of genuine antibody rather than an artefact or a reactivation marker.

IgG rises later than IgM and persists, so IgG without IgM signifies past infection and often immunity. This is the pattern used to define immune status.

Susceptibility would show neither antibody; acute infection would show IgM; and the result is a normal mature response, not an artefact.

Sanger sequencing reads the consensus of the polymerase chain reaction (PCR) amplified template population, so a minority variant appears on the chromatogram as a secondary peak only when it reaches about 20% or more of the population.

Below that threshold it is lost in the wild-type signal. This is why next-generation (deep) sequencing, which can detect variants at about 1% or lower, is increasingly used in research and reference settings. The lower thresholds of about 1% and 5% overstate Sanger sensitivity, while 50% and 95% understate it.

Antibody from blood crosses a damaged blood-brain barrier, so CSF antibody is diagnostic only when intrathecal synthesis is shown, by comparing the CSF-to-serum ratio of specific antibody against that of total IgG or albumin.

Isotype, assay method, titre and fever do not by themselves distinguish local production from passive transfer.

LAMP is isothermal, running at a single temperature, so it needs no thermal cycler and can be read simply by turbidity or a dye, which makes it attractive near the patient.

It does not need a thermal cycler or capillary electrophoresis, does not use radioactive labels, and can be adapted to RNA and crude samples.

Incorporating uracil in place of thymine and adding uracil-N-glycosylase destroys any carried-over uracil-containing amplicon before the next reaction, specifically countering carryover contamination.

Bovine serum albumin and extra polymerase counter inhibition, higher annealing temperature raises specificity, and neither magnesium nor sample volume addresses carryover.

The branched DNA assay leaves the target untouched and builds a tree of labelled probes onto it, amplifying the signal in proportion to target. This avoids amplicon carryover and gives stable quantification.

PCR, NASBA, LAMP and TMA are all target-amplification methods: they make more copies of the target sequence itself.

The copy-to-unit relationship differs between assays, so reporting against a WHO international standard in IU/mL lets a load be compared across laboratories and platforms and lets serial results be tracked meaningfully.

Copies can be measured, international units are not intrinsically more sensitive, the scale is not limited to DNA viruses, and quantification still uses a standard curve.

SYBR Green binds any double-stranded DNA, so it lights up primer dimers and off-target products as readily as the intended amplicon. Melting-curve analysis then resolves products by their distinct melting points, restoring specificity.

The dye is not sequence-specific, does not quench a probe, does not read RNA, and fluoresces when bound to double-stranded DNA rather than above the melting point.

By capturing the patient’s IgM first, the format removes IgG before antigen is added, so it avoids both the rheumatoid-factor false positive and the false negative from competing specific IgG.

It is not chosen for cost or speed, it targets IgM specifically rather than IgG, it still requires viral antigen, and it does not measure avidity.

a. The discordance most likely represents a false-positive screen. The screening assay is built for sensitivity, so it produces some false positives, and in a person with low pre-test probability a true positive is unlikely; the specific confirmatory assay is the more trustworthy result here.

b. The usual explanation is cross-reactivity or non-specific binding in the sensitive screen. The alternative that must be excluded is very early infection within the window, when antibody is only beginning to appear and the confirmatory assay has not yet turned positive.

c. Repeat testing on a fresh sample and a later follow-up sample, and resolve with a more specific confirmatory assay or a molecular test; interpret throughout against the clinical picture rather than acting on the screen alone.