Serological Diagnosis — MCQs
What condition is known to cause a false positive ELISA for HIV?
A patient was suspected of having brucellosis. A serum sample was sent for a standard agglutination test, which was initially negative but became positive after dilution of the sample. What is the most likely reason for the initial negative test?
Which of the following methods can be used to detect congenital rubella infection in the fetus?
To determine the endemicity of hepatitis B, what should be measured?
Which of the following statements about the Widal test is true?
All are true about Widal test EXCEPT:
Screening test for HIV infection in a patient prior to the development of antibodies (in window period):
Best method to diagnose HIV in an infant?
Sabin Feldman dye test is used for diagnosis of which of the following condition?
A female patient presents with dysuria and frequency. A coagulase-negative, novobiocin-resistant Staphylococcus species (>10^4 CFU/mL) was grown in urine culture. What does this indicate?
Serological Diagnosis Indian Medical PG Practice Questions and MCQs
Question 1: What condition is known to cause a false positive ELISA for HIV?
- A. Multiple myeloma
- B. Lepromatous leprosy
- C. Systemic lupus erythematosus (SLE) (Correct Answer)
- D. None of the listed conditions
Explanation: ***Systemic lupus erythematosus (SLE)*** - Patients with **SLE** can produce a variety of **autoantibodies**, some of which may cross-react with components of the HIV ELISA assay, leading to a false positive result [1]. - The immune dysregulation in SLE can cause **non-specific B-cell activation** and antibody production that confounds serological tests [1]. *Multiple myeloma* - This condition involves the uncontrolled proliferation of **plasma cells** and the overproduction of a single type of **monoclonal immunoglobulin** (M-protein). - While it can lead to various immune dysfunctions, it is not typically associated with false positive HIV ELISA results. *Lepromatous leprosy* - This severe form of leprosy is characterized by a **weak cell-mediated immune response** and high bacterial load, leading to widespread infection and hypergammaglobulinemia. - Though it involves substantial immune system changes, it is not a common cause of false positive HIV ELISA results. *None of the listed conditions* - This option is incorrect because **Systemic lupus erythematosus (SLE)** is a well-documented cause of false positive HIV ELISA results due to its complex autoimmune pathology [1].
Question 2: A patient was suspected of having brucellosis. A serum sample was sent for a standard agglutination test, which was initially negative but became positive after dilution of the sample. What is the most likely reason for the initial negative test?
- A. Antigen antibody complexes
- B. Postzone phenomenon
- C. Complement inactivation
- D. Prozone phenomenon (Correct Answer)
Explanation: ***Correct: Prozone phenomenon*** - The **prozone phenomenon** occurs when there is a very high concentration of antibodies in the patient's serum, leading to the formation of small antigen-antibody complexes that do not agglutinate or precipitate. - Diluting the sample reduces the antibody concentration, allowing for optimal antigen-antibody lattice formation and visible agglutination. - This is the classic explanation for a **negative test becoming positive after dilution** in brucellosis serology. *Incorrect: Antigen antibody complexes* - While agglutination tests rely on the formation of **antigen-antibody complexes**, the initial negative result despite a positive finding after dilution indicates a specific issue with complex *visibility* or *stability* rather than the general presence of complexes. - This option is too general and doesn't explain why dilution would change the result from negative to positive. *Incorrect: Postzone phenomenon* - The **postzone phenomenon** occurs when there is an *excess of antigen* relative to antibody, leading to no visible agglutination. - In such a case, diluting the sample (which would reduce antigen concentration or keep antibody concentration too low) would typically *not* lead to a positive result; in fact, further dilution of antibodies would worsen the outcome. - Postzone is the opposite mechanism and would not be corrected by dilution. *Incorrect: Complement inactivation* - **Complement inactivation** is not directly relevant to the mechanism of agglutination tests, which primarily depend on direct antibody-antigen binding for visible clumping. - These tests do not typically require complement activity for their primary reaction, nor are they inhibited by complement inactivation.
Question 3: Which of the following methods can be used to detect congenital rubella infection in the fetus?
- A. IgA Antibody in fetal blood
- B. IgM antibody in fetal blood (Correct Answer)
- C. Fetal hemoglobin
- D. T4 cell count
Explanation: ***IgM antibody in fetal blood*** - **IgM antibodies** are the first antibodies produced in response to an infection and do not cross the **placental barrier**. - Their presence in fetal blood indicates that the fetus has mounted its own immune response to an infection, such as **rubella**. *IgA Antibody in fetal blood* - **IgA antibodies** are primarily found in mucous secretions and are not routinely used for diagnosing congenital infections in fetal blood. - While IgA can be produced by the fetus, **IgM** is the more definitive marker for acute fetal infection. *Fetal hemoglobin* - **Fetal hemoglobin (HbF)** is a normal component of fetal blood and its presence is not indicative of an infection. - HbF levels can be used to assess fetal anemia or certain hemoglobinopathies, but not infectious diseases. *T4 cell count* - **T4 cell count** (CD4+ T cells) is a measure of immune system function, often used in conditions like HIV. - It does not directly detect the presence of the rubella virus or antibodies against it.
Question 4: To determine the endemicity of hepatitis B, what should be measured?
- A. HBsAg (Hepatitis B surface antigen) (Correct Answer)
- B. HBcAg (Hepatitis B core antigen)
- C. HBeAg (Hepatitis B e antigen)
- D. Anti-HBeAg (Hepatitis B e antibody)
Explanation: ***HBsAg*** - **HBsAg (Hepatitis B surface antigen)** is the primary marker used to determine the **endemicity** of hepatitis B because its persistent presence indicates **chronic infection**. - A high prevalence of HBsAg in a population signifies a high burden of chronic hepatitis B infection, reflecting the endemic nature of the disease in that region. *HBcAg (Hepatitis B core antigen)* - **HBcAg** is an **intracellular antigen** and is not detectable in the serum, making it unsuitable for population-level screening or endemicity assessment. - While important for viral replication, its absence in routine blood tests means it cannot be used to gauge the prevalence of infection in a community. *HBeAg (Hepatitis B e antigen)* - **HBeAg** indicates **active viral replication** and high infectivity but is not the best marker for overall endemicity. - A positive HBeAg suggests active disease and high transmissibility in an infected individual, but not the general prevalence of chronic infection in a population. *Anti-HBeAg (Hepatitis B e antibody)* - **Anti-HBeAg** indicates a **decrease in viral replication** and a lower risk of transmission, often representing a stage of immune control. - While useful for monitoring disease progression, it is an antibody response and does not directly measure the presence of chronic infection or endemicity.
Question 5: Which of the following statements about the Widal test is true?
- A. The H-antigen is the most immunogenic.
- B. Felix tubes are not used in the Widal test.
- C. Anti-O antibody persists longer than anti-H antibody.
- D. The O antigen used in the Widal test is from S. typhi. (Correct Answer)
Explanation: ***Correct: The O antigen used in the Widal test is from S. typhi.*** - The Widal test uses **O (somatic) antigens from S. Typhi** to detect anti-O antibodies - It also uses **H (flagellar) antigens from S. Typhi** to detect anti-H antibodies - Additionally, antigens from **S. Paratyphi A and B** are included for comprehensive detection of enteric fever - The statement is correct that O antigen from S. typhi is used (along with antigens from other organisms) *Incorrect: The H-antigen is the most immunogenic.* - The **O antigen** is generally considered more immunogenic than the H antigen in enteric fever - Anti-O antibodies appear earlier and are more specific for acute infection - However, O antibodies disappear faster after recovery *Incorrect: Felix tubes are not used in the Widal test.* - **Dreyer's tubes** (also known as Felix tubes) are traditionally used in the Widal test - These special tubes allow for quantitative antibody titration - They enable observation of agglutination patterns at different serum dilutions *Incorrect: Anti-O antibody persists longer than anti-H antibody.* - This is **backwards** - Anti-H antibodies actually persist longer (can last for years) - **Anti-O antibodies** appear later and disappear relatively quickly after infection resolves - Anti-O antibodies are more indicative of acute/recent infection - Anti-H antibodies are less specific due to their prolonged persistence and possible cross-reactions
Question 6: All are true about Widal test EXCEPT:
- A. Detects IgG antibodies
- B. Cross-reacts with malaria
- C. O titer rises earlier than H
- D. Single high titer diagnostic (Correct Answer)
Explanation: ***Single high titer diagnostic*** - **FALSE/INCORRECT Statement** - This is the **EXCEPTION** - a single high Widal titer alone is **NOT diagnostic** for typhoid fever - Healthy individuals in endemic areas may have high background titers due to previous exposure - **Diagnostic criteria** require either: - **Four-fold rise** in antibody titers between acute and convalescent sera (2-3 weeks apart), OR - Single titer ≥1:160 (O antigen) or ≥1:160 (H antigen) **with supportive clinical features** in non-endemic areas - Blood culture remains the **gold standard** for diagnosis *Detects IgG antibodies* - TRUE - The Widal test detects both **IgM** and **IgG antibodies** against O and H antigens of *Salmonella Typhi* - **IgM antibodies** indicate recent/acute infection - **IgG antibodies** suggest past exposure, vaccination, or chronic carriage *Cross-reacts with malaria* - TRUE - The Widal test has **low specificity** and produces **false positives** due to cross-reactivity - Known cross-reactions with: **malaria**, dengue fever, non-typhoidal Salmonella infections, and other Gram-negative infections - This limitation can lead to misdiagnosis in endemic regions *O titer rises earlier than H* - TRUE - **Anti-O antibodies** (primarily IgM) appear earlier, typically within **6-8 days** after fever onset - **Anti-H antibodies** (primarily IgG) appear later but **persist longer** in serum - O antibodies indicate acute infection; H antibodies may persist for years after infection or vaccination
Question 7: Screening test for HIV infection in a patient prior to the development of antibodies (in window period):
- A. Western blot
- B. p24 antigen (Correct Answer)
- C. ELISA
- D. All of the options
Explanation: ***p24 antigen*** - The **p24 antigen** test detects a structural protein of HIV, which becomes detectable before the development of antibodies (during the **window period**). - This test is crucial for early detection of HIV infection, especially when antibody tests might still be negative due to the time lag in immune response. *Western blot* - The **Western blot** is a confirmatory test for HIV, detecting specific antibodies to various HIV proteins. - It becomes positive only after the development of antibodies, making it unsuitable for detecting infection during the **window period**. *ELISA* - **ELISA** (Enzyme-Linked Immunosorbent Assay) is a common screening test for HIV, primarily detecting **HIV antibodies**. - Like Western blot, it relies on antibody presence and thus will be negative during the **window period**. *All of the options* - This option is incorrect because both **Western blot** and **ELISA** detect antibodies, making them unsuitable for screening during the **window period**. - Only the **p24 antigen** test can detect HIV infection before antibody seroconversion.
Question 8: Best method to diagnose HIV in an infant?
- A. ELISA
- B. PCR (Correct Answer)
- C. Western blot
- D. All of the options
Explanation: ***PCR*** - **Polymerase Chain Reaction (PCR)** detects **HIV nucleic acids** (DNA or RNA) directly, which is crucial for infants because maternal antibodies can persist for up to 18 months, interfering with antibody-based tests. - PCR allows for early diagnosis, often within the first few weeks or months of life, facilitating timely intervention. *ELISA* - **Enzyme-linked immunosorbent assay (ELISA)** detects HIV antibodies. - In infants, ELISA can be misleading due to the presence of **maternal HIV antibodies** transferred across the placenta, making it unreliable for diagnosing active infection. *Western blot* - **Western blot** is used to confirm positive ELISA results in adults by detecting specific HIV proteins. - Like ELISA, it relies on the detection of **antibodies** and is therefore not reliable in infants due to maternally transmitted antibodies. *All of the options* - This option is incorrect because **ELISA** and **Western blot** are antibody-based tests that are unreliable in infants due to the presence of **maternal antibodies**. - Only **PCR** directly detects the virus itself, making it the preferred diagnostic method in this age group.
Question 9: Sabin Feldman dye test is used for diagnosis of which of the following condition?
- A. Botulism
- B. Toxoplasmosis (Correct Answer)
- C. Sarcoidosis
- D. Yellow fever
Explanation: ***Toxoplasmosis*** The **Sabin-Feldman dye test** is a **serological assay** used to detect specific IgG antibodies against *Toxoplasma gondii*, the causative agent of toxoplasmosis. It measures the ability of antibodies in a patient's serum to prevent the cytoplasmic staining of live toxoplasma tachyzoites by methylene blue dye, indicating an **active immune response** to the parasite. This classic test has high sensitivity and specificity for detecting toxoplasma antibodies and is considered the gold standard serological test, though it has been largely replaced by ELISA and IFA in routine practice due to the requirement for live organisms. *Botulism* Botulism is diagnosed through **toxin detection** in serum, stool, or food samples using mouse bioassay, or by culturing *Clostridium botulinum* from clinical specimens. The Sabin-Feldman dye test is not relevant for the diagnosis of botulism, which is a **neuroparalytic disease** caused by botulinum neurotoxin blocking acetylcholine release at neuromuscular junctions. *Sarcoidosis* Sarcoidosis is a multisystem **granulomatous disease** diagnosed primarily by **tissue biopsy** showing non-caseating granulomas along with compatible clinical and radiological findings. Supportive tests include elevated serum ACE levels and Kveim test (historical). There is no serological test like the Sabin-Feldman dye test associated with the diagnosis of sarcoidosis, as it is not an infectious disease requiring antibody detection. *Yellow fever* Yellow fever is a **viral hemorrhagic disease** caused by a flavivirus, diagnosed by detecting viral RNA through RT-PCR or specific IgM antibodies in the acute phase of infection using ELISA or immunofluorescence. The Sabin-Feldman dye test is not used for viral infections like yellow fever, as it specifically targets **toxoplasma antibodies** and has no role in arboviral disease diagnosis.
Question 10: A female patient presents with dysuria and frequency. A coagulase-negative, novobiocin-resistant Staphylococcus species (>10^4 CFU/mL) was grown in urine culture. What does this indicate?
- A. UTI (Correct Answer)
- B. Commensal
- C. Contamination
- D. Repeat culture needed
Explanation: ***UTI*** - The isolation of a **coagulase-negative, novobiocin-resistant Staphylococcus** in a patient with UTI symptoms suggests **_Staphylococcus saprophyticus_**, a common cause of UTIs in young women. - A bacterial count of **>10^4 CFU/mL** is generally considered significant for diagnosing a UTI, indicating active infection rather than contamination. - _S. saprophyticus_ accounts for 10-20% of UTIs in sexually active young women and is the second most common cause after _E. coli_. *Commensal* - While some coagulase-negative staphylococci can be commensals, **_S. saprophyticus_** is an important pathogen, especially in UTIs. - The combination of **novobiocin resistance** and a significant bacterial count in a symptomatic patient strongly points away from a commensal role. *Contamination* - **Contamination** usually involves lower bacterial counts (<10^4 CFU/mL) or the isolation of multiple different organisms. - The presence of **>10^4 CFU/mL** of a pure culture of a known urinary pathogen (_S. saprophyticus_) in a symptomatic patient makes contamination unlikely. *Repeat culture needed* - Repeat cultures are indicated when initial results are equivocal (e.g., low counts, mixed flora, or asymptomatic bacteriuria). - For symptomatic UTI with **>10^4 CFU/mL** of a known pathogen, a single culture is sufficient for diagnosis and treatment initiation. - Multiple consecutive samples are primarily used for diagnosing **bacteremia** or **endocarditis**, not routine UTI.
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