A 27-year-old woman at 11 weeks of pregnancy has Toxoplasma serology at her booking visit: IgG nonreactive (below 1 IU/mL; reactive at 3 IU/mL or more) and IgM reactive (index 1.4; reactive at 1.1 or more). She feels well and has no lymphadenopathy. A second sample taken 3 weeks later is tested in the same run as the stored first sample: IgG is nonreactive in both, and the IgM index is 1.4 in the first and 1.5 in the second. What is the best interpretation?

Nonspecific IgM; she is still not immune

Past infection, with IgG now below cutoff

Very early primary infection; treat now

Recent infection; IgG avidity will date it

A 7-week-old infant who failed newborn hearing screening has a positive urine CMV PCR. No CMV test was done in the first weeks of life, but the newborn screening dried blood spot card is stored. What is the best way to show that the infection was congenital?

CMV PCR on the stored dried blood spot

CMV IgG avidity on the infant's serum

Report congenital CMV from the urine PCR

In an IgG avidity assay, antigen-antibody complexes are washed with urea before detection. The purpose of the urea is to:

Elute weakly bound IgG from recent infection

Block heterophile antibodies in the serum

Destroy rheumatoid factor that gives false IgG

Remove IgM that competes with IgG for antigen

The heterophile antibodies of infectious mononucleosis are mainly:

IgM antibodies that agglutinate horse red cells

IgG antibodies to EBV nuclear antigen

IgM antibodies to EBV viral capsid antigen

IgA antibodies to EBV early antigen

A 17-year-old has had fever, sore throat and posterior cervical lymphadenopathy for 4 days, and the blood film shows many reactive lymphocytes. A rapid heterophile test is negative. What is the best next step?

Test EBV VCA IgM or repeat the heterophile later

Repeat the heterophile test on serial serum dilutions

Test for EBNA antibody to confirm acute infection

Report that infectious mononucleosis has been excluded

TORCH, EBV and other viral serology

9 DOH Abu Dhabi (HAAD) lab licensing practice questions on TORCH, EBV and other viral serology, part of Immunology and Serology. The first 9 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 27-year-old woman at 11 weeks of pregnancy has Toxoplasma serology at her booking visit: IgG nonreactive (below 1 IU/mL; reactive at 3 IU/mL or more) and IgM reactive (index 1.4; reactive at 1.1 or more). She feels well and has no lymphadenopathy. A second sample taken 3 weeks later is tested in the same run as the stored first sample: IgG is nonreactive in both, and the IgM index is 1.4 in the first and 1.5 in the second. What is the best interpretation?
1. APast infection, with IgG now below cutoff
2. BVery early primary infection; treat now
3. CRecent infection; IgG avidity will date it
4. DNonspecific IgM; she is still not immune
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Answer: D · Nonspecific IgM; she is still not immune
In a true primary Toxoplasma infection, IgG appears within about 1 to 2 weeks of IgM, so by 3 weeks it should have seroconverted. IgM that stays at the same low level with no IgG at all, in two samples tested side by side, is a nonspecific (false-positive) IgM, and she has not been infected. Avidity measures how strongly IgG binds, so it cannot be done when there is no IgG, and a past infection leaves lasting IgG rather than isolated IgM. She remains susceptible and should be advised to avoid undercooked meat, unwashed produce and cat litter. A reference laboratory can confirm the result if there is doubt, and some programs retest seronegative women later in pregnancy.
- A: Past infection leaves lasting IgG, not isolated IgM.
- B: A true infection would have produced IgG within 3 weeks; none appeared.
- C: Avidity measures IgG binding and cannot be done without IgG.
Question 2 · Hard
A 7-week-old infant who failed newborn hearing screening has a positive urine CMV PCR. No CMV test was done in the first weeks of life, but the newborn screening dried blood spot card is stored. What is the best way to show that the infection was congenital?
1. ACMV PCR on the stored dried blood spot
2. BCMV IgG avidity on the infant's serum
3. CReport congenital CMV from the urine PCR
4. DCMV IgG on the infant's serum
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Answer: A · CMV PCR on the stored dried blood spot
Congenital CMV is diagnosed by detecting virus in urine or saliva within the first 2 to 3 weeks of life. After that, a positive test could reflect infection acquired at birth or from breast milk, which rarely causes hearing loss. CMV DNA in the dried blood spot taken for newborn screening shows that virus was present at birth, although a negative spot does not exclude congenital infection. Infant CMV IgG mostly reflects maternal antibody and cannot separate the two.
- B: Avidity times infection in the mother and cannot date infection in the infant.
- C: After 3 weeks of age a positive urine PCR cannot separate congenital from postnatal infection.
- D: Infant IgG in the first months is largely maternal antibody.
Question 3 · Medium
In an IgG avidity assay, antigen-antibody complexes are washed with urea before detection. The purpose of the urea is to:
1. AElute weakly bound IgG from recent infection
2. BBlock heterophile antibodies in the serum
3. CDestroy rheumatoid factor that gives false IgG
4. DRemove IgM that competes with IgG for antigen
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Answer: A · Elute weakly bound IgG from recent infection
Early in infection IgG binds antigen with low avidity; over months, affinity maturation produces IgG that binds firmly. A chaotropic agent such as urea strips off low-avidity IgG but not high-avidity IgG, and the avidity index compares the signal with and without urea. A high index excludes infection within a kit-defined period, often about 3 to 4 months. A low index is less useful, because low avidity can persist for months in some people.
- B: Heterophile antibodies are handled with blocking reagents, not urea washes.
- C: Rheumatoid factor interference is removed by IgG absorption, not by urea.
- D: Avidity measures how firmly IgG binds; removing IgM is not the purpose of the urea wash.
Question 4 · Easy
The heterophile antibodies of infectious mononucleosis are mainly:
1. AIgG antibodies to EBV nuclear antigen
2. BIgM antibodies that agglutinate horse red cells
3. CIgM antibodies to EBV viral capsid antigen
4. DIgA antibodies to EBV early antigen
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Answer: B · IgM antibodies that agglutinate horse red cells
Heterophile antibodies are IgM antibodies made during EBV infection that react with antigens on red cells of other species, such as sheep, horse and ox, but are not directed at EBV itself. Rapid slide tests detect them by agglutination of horse red cells or by immunoassay with bovine red cell antigen. They appear in the first weeks of illness and fade over a few months. EBV-specific antibodies such as VCA IgM and EBNA antibody are measured separately when the heterophile test is negative.
- A: EBNA antibody is EBV specific and appears months after infection.
- C: VCA IgM is EBV specific; heterophile antibodies do not bind EBV antigens.
- D: Early antigen antibodies are EBV specific and of variable diagnostic value.
Question 5 · Hard
A 17-year-old has had fever, sore throat and posterior cervical lymphadenopathy for 4 days, and the blood film shows many reactive lymphocytes. A rapid heterophile test is negative. What is the best next step?
1. ARepeat the heterophile test on serial serum dilutions
2. BTest for EBNA antibody to confirm acute infection
3. CReport that infectious mononucleosis has been excluded
4. DTest EBV VCA IgM or repeat the heterophile later
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Answer: D · Test EBV VCA IgM or repeat the heterophile later
Heterophile antibodies may not reach detectable levels in the first week of illness, so an early negative test is common even in typical infectious mononucleosis. When suspicion is high, EBV-specific serology (VCA IgM and IgG with EBNA antibody) gives an answer now, or the heterophile test can be repeated after about a week. EBNA antibody appears only weeks to months after onset, so a positive result points to past rather than acute infection. A single early negative heterophile test does not exclude EBV.
- A: An early negative result reflects a low antibody level, not a prozone.
- B: EBNA antibody appears late, so it marks past rather than acute infection.
- C: A single early negative heterophile test does not exclude infectious mononucleosis.
Question 6 · Medium
A teacher at 18 weeks of pregnancy was exposed 5 days ago to a child with erythema infectiosum (fifth disease). Parvovirus B19 IgG is positive and IgM is negative. What is the best interpretation?
1. AInfected from this exposure; needs fetal monitoring
2. BSusceptible; repeat both tests in 4 weeks
3. CRecent infection whose IgM has already cleared
4. DImmune from earlier infection; no further testing
Answer and explanation in LearnFree account
Question 7 · Hard
A mother seroconverted to Toxoplasma at 26 weeks of pregnancy. At birth her baby has Toxoplasma IgG positive, with IgM and IgA negative. The infant receives no treatment, and at 12 months the IgG is negative. What is the best interpretation?
1. AUninterpretable without an IgG avidity on the infant
2. BCongenital infection, shown by IgG present at birth
3. CCongenital infection; IgM and IgA were false negatives
4. DNo congenital infection; the IgG at birth was maternal
Answer and explanation in LearnFree account
Question 8 · Medium
Why is a reactive Toxoplasma IgM result, on its own, weak evidence that infection began during the current pregnancy?
1. AToxoplasma IgM can persist for a year or longer
2. BToxoplasma IgM is made only when cysts reactivate
3. CPregnancy itself often induces Toxoplasma IgM
4. DToxoplasma IgM appears only after IgG is present
Answer and explanation in LearnFree account
Question 9 · Hard
A woman with a history of recurrent genital sores, none present now, has type-specific serology: HSV-1 IgG negative; HSV-2 IgG (glycoprotein G-based EIA) index 1.6, with a positive cut-off of 1.1. What is the best next step?
1. ARepeat the same EIA when sores reappear
2. BReport HSV-2 infection on this result alone
3. CTest HSV IgM to confirm a recent infection
4. DConfirm with a second, different HSV-2 antibody test
Answer and explanation in LearnFree account

TORCH, EBV and other viral serology

A 7-week-old infant who failed newborn hearing screening has a positive urine CMV PCR. No CMV test was done in the first weeks of life, but the newborn screening dried blood spot card is stored. What is the best way to show that the infection was congenital?

In an IgG avidity assay, antigen-antibody complexes are washed with urea before detection. The purpose of the urea is to:

The heterophile antibodies of infectious mononucleosis are mainly:

A 17-year-old has had fever, sore throat and posterior cervical lymphadenopathy for 4 days, and the blood film shows many reactive lymphocytes. A rapid heterophile test is negative. What is the best next step?

A teacher at 18 weeks of pregnancy was exposed 5 days ago to a child with erythema infectiosum (fifth disease). Parvovirus B19 IgG is positive and IgM is negative. What is the best interpretation?

A mother seroconverted to Toxoplasma at 26 weeks of pregnancy. At birth her baby has Toxoplasma IgG positive, with IgM and IgA negative. The infant receives no treatment, and at 12 months the IgG is negative. What is the best interpretation?

Why is a reactive Toxoplasma IgM result, on its own, weak evidence that infection began during the current pregnancy?

A woman with a history of recurrent genital sores, none present now, has type-specific serology: HSV-1 IgG negative; HSV-2 IgG (glycoprotein G-based EIA) index 1.6, with a positive cut-off of 1.1. What is the best next step?