Rapid Diagnostic Tests — MCQs
HBsAg is based on which principle
What is the most appropriate initial investigation for a patient presenting with a history of sore throat, diarrhea, and sexual contact two weeks prior to assess for HIV?
Rapid examination of tubercle bacilli is possible with:
Most sensitive test for detecting microfilariae?
A patient with cough was sputum AFB negative but chest X-ray was suggestive of TB. What should be the next step according to RNTCP?
A 28-year-old patient presents with high-grade fever, severe cough, and progressive dyspnea. The patient lives near a poultry farm where there has been a recent bird flu outbreak. What is the most appropriate diagnostic test?
A frequent traveler presented with 4 days of continuous fever, abdominal pain, and bradycardia. What is the best diagnostic test to confirm the pathogen?
What is the primary use of Elek's Gel test?
Which of the following statements about nucleic acid amplification tests (NAATs) for STIs is FALSE?
Which of the following is a xenodiagnostic method?
Rapid Diagnostic Tests Indian Medical PG Practice Questions and MCQs
Question 1: HBsAg is based on which principle
- A. Chemiluminescence
- B. Immunofluorescence
- C. Immunochromatography assays
- D. ELISA (Correct Answer)
Explanation: ***ELISA*** - **Enzyme-linked immunosorbent assay (ELISA)** is a widely used laboratory test to detect and quantify antigens (like HBsAg) or antibodies in a sample. - It involves an enzyme-linked antibody that reacts with a substrate to produce a detectable signal, making it highly sensitive and specific for **HBsAg detection**. *Immunochromatography assays* - These are typically **rapid diagnostic tests (RDTs)** that provide quick qualitative results, often used for point-of-care testing. - While they can detect HBsAg, they generally have lower sensitivity and specificity compared to ELISA. *Chemiluminescence* - This is a detection method used in some immunoassays where a chemical reaction emits light, often providing higher sensitivity than colorimetric detection. - While it can be incorporated into HBsAg testing platforms, it is a *detection principle* rather than the primary assay principle like ELISA itself. *Immunofluorescence* - This technique uses **fluorescently labeled antibodies** to visualize antigens in cells or tissues under a fluorescence microscope [1]. - It is used for localization and identification of antigens, but not typically the primary method for routine quantitative HBsAg serology [1]. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 259-260.
Question 2: What is the most appropriate initial investigation for a patient presenting with a history of sore throat, diarrhea, and sexual contact two weeks prior to assess for HIV?
- A. Western blot
- B. Lymph node biopsy
- C. p24 antigen assay
- D. ELISA (Correct Answer)
Explanation: ***ELISA*** - **Enzyme-linked immunosorbent assay (ELISA)** is the **most appropriate initial screening test** for HIV due to its high sensitivity and ability to detect both HIV antibodies and p24 antigen in fourth-generation tests [1]. - The patient's symptoms (sore throat, diarrhea) and recent sexual contact are suggestive of acute retroviral syndrome, for which an ELISA testing for **HIV antibodies and p24 antigen** would be effective. *p24 antigen assay* - While the **p24 antigen assay** is useful for detecting HIV early (during the acute phase before antibody seroconversion), it is typically part of a **fourth-generation ELISA test**, making the stand-alone p24 assay less comprehensive as an initial investigation. - A standalone p24 antigen assay could miss the infection if the patient has already developed antibodies but not enough antigen for detection or during the window period when antibodies are rising and antigen may be declining. *Western blot* - **Western blot** is a **confirmatory test** for HIV, used to verify positive ELISA results, not as an initial screening tool [1]. - It detects specific HIV antibodies and is more expensive and labor-intensive, making it unsuitable for first-line screening [1]. *Lymph node biopsy* - **Lymph node biopsy** is an invasive procedure and is not used for routine HIV diagnosis. - It might be considered in cases of unexplained **lymphadenopathy** to rule out other conditions, but not as the initial investigation for HIV [2].
Question 3: Rapid examination of tubercle bacilli is possible with:
- A. Ziehl–Neelsen method for AFB detection.
- B. Auramine-Rhodamine fluorescent stain. (Correct Answer)
- C. Giemsa stain for blood smears.
- D. Kinyoun cold acid-fast stain.
Explanation: ***Auramine-Rhodamine fluorescent stain*** - This method provides a **more rapid** means of examining tubercle bacilli because it allows for the use of lower magnifications (e.g., 20x or 40x), enabling the viewing of a larger field and faster scanning of smears. - The **fluorescent bacilli** appear as bright yellow-green rods against a dark background, making them easier and quicker to detect compared to conventional light microscopy. *Ziehl–Neelsen method for AFB detection* - While it is a standard method for identifying **acid-fast bacilli (AFB)**, it requires higher magnification (100x oil immersion) and more extensive scanning to detect the red bacilli, making it **slower** than fluorescent methods. - It uses a basic fuchsin stain and heat to drive the stain into the waxy mycobacterial cell wall, followed by decolorization with acid-alcohol. *Giemsa stain for blood smears* - This stain is primarily used for identifying **blood parasites** (e.g., malaria) and cellular morphology in **hematological disorders**, and is not suitable for detecting mycobacteria. - It stains nuclear and cytoplasmic components differently, providing morphological details of blood cells and pathogens. *Kinyoun cold acid-fast stain* - Similar to the Ziehl-Neelsen stain, the Kinyoun method is also a conventional acid-fast stain that does not involve heat, but it still requires **high magnification** and **slow scanning** for detection. - It uses a higher concentration of basic fuchsin and a wetting agent to penetrate the cell wall without heating, but it is not considered rapid.
Question 4: Most sensitive test for detecting microfilariae?
- A. Membrane filtration technique (Correct Answer)
- B. Diethylcarbamazine (DEC) challenge test
- C. Fluorescence-based immunoassay
- D. Thick blood smear
Explanation: ***Membrane filtration technique*** - The **membrane filtration technique** is considered the most sensitive test for detecting **microfilariae** because it concentrates microfilariae from a larger volume of blood (typically 1 mL or more) onto a filter membrane, increasing detection rates, especially in low-parasite density infections. - This method physically traps the microfilariae, allowing for microscopic examination of the concentrated sample after staining, which enhances visualization. *Diethylcarbamazine (DEC) challenge test* - The **DEC challenge test** uses **diethylcarbamazine** to provoke the release of microfilariae into the peripheral blood, especially in cases of occult filariasis or when microfilaria numbers are low. - While it can be useful in certain diagnostic situations, it is **less sensitive** than membrane filtration for directly detecting circulating microfilariae and carries the risk of inducing severe adverse reactions due to rapid parasite killing. *Fluorescence-based immunoassay* - **Fluorescence-based immunoassays** detect **antigens** or **antibodies** related to filarial infection, providing evidence of exposure or active infection. - While valuable for diagnosis, especially in antibody detection for chronic or occult infections, they do not directly detect live microfilariae and thus are not the most sensitive method for *detecting microfilariae themselves*. *Thick blood smear* - A **thick blood smear** is a common and quick method for detecting microfilariae by examining a drop of blood for their presence. - However, it is **less sensitive** than the membrane filtration technique, particularly in persons with low microfilaremia, as it examines a much smaller volume of blood.
Question 5: A patient with cough was sputum AFB negative but chest X-ray was suggestive of TB. What should be the next step according to RNTCP?
- A. Nucleic acid amplification test (Correct Answer)
- B. Tuberculin test
- C. Line probe assay
- D. Culture
Explanation: Nucleic acid amplification test - According to the and Revised National Tuberculosis Control Program (RNTCP) guidelines, if sputum AFB microscopy is negative but clinical suspicion and chest X-ray point towards TB, NAAT (Nucleic Acid Amplification Test) is recommended as the next confirmatory step [1]. - NAATs like CBNAAT (Cartridge-Based Nucleic Acid Amplification Test) or TrueNat provide rapid detection of Mycobacterium tuberculosis and resistance to Rifampicin, aiding in early diagnosis and appropriate treatment initiation. Tuberculin test - The Tuberculin Skin Test (TST), also known as the Mantoux test, indicates past exposure to TB or latent infection, but it cannot differentiate between active disease, latent infection, or past treated infection [2]. - A positive TST in an adult with a suggestive chest X-ray still requires further investigation for active disease, as it does not confirm active pulmonary TB [2]. Line probe assay - Line Probe Assay (LPA) is a molecular test used for rapid detection of MDR-TB (multi-drug resistant TB) by identifying mutations associated with resistance to Rifampicin and Isoniazid. - While useful for resistance testing, it typically requires a positive culture or direct sputum sample with a higher bacterial load and is not the primary diagnostic test for initial confirmation of TB when sputum AFB is negative. Culture - Mycobacterial culture is the gold standard for TB diagnosis, providing definitive confirmation and enabling drug susceptibility testing (DST) [1]. - However, culture results can take several weeks (typically 3-6 weeks), which delays treatment initiation, making it a less immediate next step compared to rapid molecular tests like NAAT in cases of strong clinical suspicion.
Question 6: A 28-year-old patient presents with high-grade fever, severe cough, and progressive dyspnea. The patient lives near a poultry farm where there has been a recent bird flu outbreak. What is the most appropriate diagnostic test?
- A. Virus isolation
- B. Rapid antigen test
- C. RT-PCR (Correct Answer)
- D. Antibody detection
Explanation: ***RT-PCR*** - **Reverse transcription polymerase chain reaction (RT-PCR)** is the gold standard for diagnosing avian influenza (bird flu) due to its high sensitivity and specificity. - It directly detects the **viral RNA** and can confirm active infection, which is crucial given the recent bird flu outbreak and the patient's symptoms. *Virus isolation* - **Virus isolation** is a highly specific method but is time-consuming and requires specialized laboratory facilities, making it less suitable for rapid diagnosis in an acute clinical setting. - It is often used for **viral characterization** and surveillance rather than initial patient diagnosis. *Rapid antigen test* - **Rapid antigen tests** for influenza generally have lower sensitivity for avian influenza compared to RT-PCR, leading to a higher risk of false negatives. - While quick, their reduced accuracy might delay appropriate patient management, especially in a potential outbreak scenario. *Antibody detection* - **Antibody detection** (serology) indicates past exposure or infection, as antibodies typically take several days to weeks to develop. - It is not suitable for diagnosing an **acute, active infection** like the current presentation of fever, headache, and rash due to the lag in antibody production.
Question 7: A frequent traveler presented with 4 days of continuous fever, abdominal pain, and bradycardia. What is the best diagnostic test to confirm the pathogen?
- A. Widal test
- B. Blood culture (Correct Answer)
- C. Urine culture
- D. Stool culture
Explanation: ***Blood culture*** - **Blood culture** is the most sensitive and specific test for confirming **typhoid fever** in the first week of illness. - The presence of **continuous fever** (step-ladder pattern), **abdominal pain**, and **relative bradycardia** in a traveler strongly suggests typhoid fever caused by *Salmonella Typhi*. *Widal test* - The **Widal test** detects antibodies against *Salmonella Typhi* antigens and is often positive later in the disease course. - It has **limited sensitivity and specificity**, especially in endemic areas or with prior vaccination, leading to false positives and negatives. *Urine culture* - **Urine culture** has a low yield for *Salmonella Typhi*, as bacteria are intermittently shed in urine, usually later in the disease. - It's primarily useful for diagnosing **urinary tract infections** or in chronic carriers of typhoid. *Stool culture* - **Stool culture** yield is higher in the later stages of typhoid fever, as *Salmonella Typhi* is shed in feces. - Its sensitivity is lower than blood culture in the early acute phase when bacteremia is most prominent.
Question 8: What is the primary use of Elek's Gel test?
- A. Influenza
- B. Diphtheria (Correct Answer)
- C. Brucellosis
- D. Cholera
Explanation: ***Diphtheria*** - **Elek's Gel immunodiffusion test** is a crucial in vitro assay used to detect **toxin production** by *Corynebacterium diphtheriae* strains - This test helps differentiate **toxigenic strains** (which cause diphtheria) from **non-toxigenic strains** - Based on the principle of **immunodiffusion** where antitoxin-impregnated filter paper is placed on agar and bacterial strains are streaked perpendicular to it - **Lines of precipitation** form if the strain produces diphtheria toxin *Influenza* - Influenza diagnosis primarily relies on rapid antigen detection tests, PCR, or viral culture - The Elek test is not used for the identification or toxin detection of *Influenza virus* *Brucellosis* - Brucellosis diagnosis is typically confirmed by serological tests (e.g., agglutination tests) or blood cultures - The Elek test has no role in the diagnosis of *Brucella* infections *Cholera* - Cholera is diagnosed by identifying *Vibrio cholerae* in stool samples through culture and biochemical tests - The Elek test is irrelevant to the diagnosis of cholera
Question 9: Which of the following statements about nucleic acid amplification tests (NAATs) for STIs is FALSE?
- A. They can be used for test of cure after 3 weeks
- B. They can detect dead organisms after treatment
- C. They can be used for pharyngeal gonorrhea screening
- D. They are less sensitive than culture for rectal chlamydia (Correct Answer)
Explanation: ***They are less sensitive than culture for rectal chlamydia*** - This statement is **FALSE**. NAATs are generally **more sensitive** than culture methods for detecting *Chlamydia trachomatis* in all anatomical sites, including the rectum. - The high sensitivity of NAATs allows for the detection of very low bacterial loads, making them the preferred diagnostic method for many STIs. *They can be used for test of cure after 3 weeks* - This statement is generally **true**. While a "test of cure" (TOC) is not routinely recommended for uncomplicated *Chlamydia* or *Gonorrhea* infections due to high treatment efficacy, it can be considered in specific circumstances (e.g., persistent symptoms, pregnancy, or use of alternative regimens). - If a TOC is performed, it should ideally be done **no sooner than 3 weeks post-treatment** to minimize potential false positives from detecting residual nucleic acids from dead organisms. *They can detect dead organisms after treatment* - This statement is **true**. NAATs detect the **nucleic acids (DNA or RNA)** of the target organism. - These nucleic acids can persist in the body for a period even after the organism has been killed by treatment, leading to a positive NAAT result despite successful eradication of the infection. *They can be used for pharyngeal gonorrhea screening* - This statement is **true**. NAATs are the **recommended method** for detecting *Neisseria gonorrhoeae* in extragenital sites, including the pharynx. - Pharyngeal gonorrhea is often **asymptomatic**, making screening of at-risk individuals important for public health.
Question 10: Which of the following is a xenodiagnostic method?
- A. Intradermal test on guinea pigs for toxigenicity of Corynebacterium diphtheriae
- B. Injecting a hamster with splenic biopsy for diagnosis of leishmaniasis
- C. Injecting Aedes thorax with blood of a suspected dengue patient (Correct Answer)
- D. Rabbit ileal loop for enterotoxigenic Escherichia coli
Explanation: ***Injecting Aedes thorax with blood of a suspected dengue patient*** - **Xenodiagnosis** involves using a live arthropod vector (such as an *Aedes* mosquito) to detect the presence of pathogens in a host by feeding the vector on the host and subsequently examining the vector for infection. - In this method, the mosquito acts as a biological incubator or amplifier for the dengue virus, which can then be detected within the mosquito, indicating infection in the patient. *Intradermal test on guinea pigs for toxigenicity of Corynebacterium diphtheria* - This is an **animal pathogenicity test** to determine if the *Corynebacterium diphtheriae* strain produces toxin, but it is not xenodiagnosis as the animal is the test subject, not a vector for detection. - The test assesses the virulence of the bacterium directly in the animal, rather than using the animal to detect an existing infection in another host. *Injecting a hamster with splenic biopsy for diagnosis of leishmaniasis* - This method is an example of **animal inoculation** or **culture in vivo**, where a susceptible animal is used to grow and amplify a suspected pathogen from a patient sample. - While it uses an animal for diagnosis, it's not xenodiagnosis because the hamster is not a natural vector that feeds on the patient to acquire the pathogen. *Rabbit ileal loop for enterotoxigenic Escherichia coli* - This is an **animal model** used to detect the production of **enterotoxins** by *Escherichia coli*, which cause fluid accumulation in the rabbit ileum. - This method tests for the pathogenic effect of the bacteria rather than using an arthropod vector to detect infection from a human host.
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free