Sensitivity/Specificity

Sensitivity/Specificity US Medical PG Question 1: A scientist in Chicago is studying a new blood test to detect Ab to EBV with increased sensitivity and specificity. So far, her best attempt at creating such an exam reached 82% sensitivity and 88% specificity. She is hoping to increase these numbers by at least 2 percent for each value. After several years of work, she believes that she has actually managed to reach a sensitivity and specificity much greater than what she had originally hoped for. She travels to China to begin testing her newest blood test. She finds 2,000 patients who are willing to participate in her study. Of the 2,000 patients, 1,200 of them are known to be infected with EBV. The scientist tests these 1,200 patients' blood and finds that only 120 of them tested negative with her new exam. Of the patients who are known to be EBV-free, only 20 of them tested positive. Given these results, which of the following correlates with the exam's specificity?

• A. 82%
• B. 90%
• C. 84%
• D. 86%
• E. 98% (Correct Answer)

Sensitivity/Specificity Explanation: ***98%*** - **Specificity** measures the proportion of **true negatives** among all actual negatives. - In this case, 800 patients are known to be EBV-free (actual negatives), and 20 of them tested positive (false positives). This means 800 - 20 = 780 tested negative (true negatives). Specificity = (780 / 800) * 100% = **98%**. *82%* - This value represents the *original sensitivity* before the scientist’s new attempts to improve the test. - It does not reflect the *newly calculated specificity* based on the provided data. *90%* - This value represents the *newly calculated sensitivity* of the test, not the specificity. - Out of 1200 EBV-infected patients, 120 tested negative (false negatives), meaning 1080 tested positive (true positives). Sensitivity = (1080 / 1200) * 100% = 90%. *84%* - This percentage is not directly derived from the information given for either sensitivity or specificity after the new test results. - It does not correspond to any of the calculated values for the new test's performance. *86%* - This percentage is not directly derived from the information given for either sensitivity or specificity after the new test results. - It does not correspond to any of the calculated values for the new test's performance.

Sensitivity/Specificity US Medical PG Question 2: A researcher is trying to determine whether a newly discovered substance X can be useful in promoting wound healing after surgery. She conducts this study by enrolling the next 100 patients that will be undergoing this surgery and separating them into 2 groups. She decides which patient will be in which group by using a random number generator. Subsequently, she prepares 1 set of syringes with the novel substance X and 1 set of syringes with a saline control. Both of these sets of syringes are unlabeled and the substances inside cannot be distinguished. She gives the surgeon performing the surgery 1 of the syringes and does not inform him nor the patient which syringe was used. After the study is complete, she analyzes all the data that was collected and performs statistical analysis. This study most likely provides which level of evidence for use of substance X?

• A. Level 3
• B. Level 1 (Correct Answer)
• C. Level 4
• D. Level 5
• E. Level 2

Sensitivity/Specificity Explanation: ***Level 1*** - The study design described is a **randomized controlled trial (RCT)**, which is considered the **highest level of evidence (Level 1)** in the hierarchy of medical evidence. - Key features like **randomization**, **control group**, and **blinding (double-blind)** help minimize bias and strengthen the validity of the findings. *Level 2* - Level 2 evidence typically comprises **well-designed controlled trials without randomization** (non-randomized controlled trials) or **high-quality cohort studies**. - While strong, they do not possess the same level of internal validity as randomized controlled trials. *Level 3* - Level 3 evidence typically includes **case-control studies** or **cohort studies**, which are observational designs and carry a higher risk of bias compared to RCTs. - These studies generally do not involve randomization or intervention assignment by the researchers. *Level 4* - Level 4 evidence is usually derived from **case series** or **poor quality cohort and case-control studies**. - These studies provide descriptive information or investigate associations without strong control for confounding factors. *Level 5* - Level 5 evidence is the **lowest level of evidence**, consisting of **expert opinion** or **animal research/bench research**. - This level lacks human clinical data or systematic investigative rigor needed for higher evidence levels.

Sensitivity/Specificity US Medical PG Question 3: You conduct a medical research study to determine the screening efficacy of a novel serum marker for colon cancer. The study is divided into 2 subsets. In the first, there are 500 patients with colon cancer, of which 450 are found positive for the novel serum marker. In the second arm, there are 500 patients who do not have colon cancer, and only 10 are found positive for the novel serum marker. What is the overall sensitivity of this novel test?

• A. 450 / (450 + 10)
• B. 490 / (10 + 490)
• C. 490 / (50 + 490)
• D. 450 / (450 + 50) (Correct Answer)
• E. 490 / (450 + 490)

Sensitivity/Specificity Explanation: ***450 / (450 + 50)*** - **Sensitivity** is defined as the proportion of actual positive cases that are correctly identified by the test. - In this study, there are **500 patients with colon cancer** (actual positives), and **450 of them tested positive** for the marker, while **50 tested negative** (500 - 450 = 50). Therefore, sensitivity = 450 / (450 + 50) = 450/500 = 0.9 or 90%. *450 / (450 + 10)* - This formula represents **Positive Predictive Value (PPV)**, which is the probability that a person with a positive test result actually has the disease. - It incorrectly uses the total number of **test positives** in the denominator (450 true positives + 10 false positives) instead of the total number of diseased individuals, which is needed for sensitivity. *490 / (10 + 490)* - This is actually the correct formula for **specificity**, not sensitivity. - Specificity = TN / (FP + TN) = 490 / (10 + 490) = 490/500 = 0.98 or 98%, which measures the proportion of actual negative cases correctly identified. - The question asks for sensitivity, not specificity. *490 / (50 + 490)* - This formula incorrectly mixes **true negatives (490)** with **false negatives (50)** in an attempt to calculate specificity. - The correct specificity formula should use false positives (10), not false negatives (50), in the denominator: 490 / (10 + 490). *490 / (450 + 490)* - This calculation incorrectly combines **true negatives (490)** and **true positives (450)** in the denominator, which does not correspond to any standard epidemiological measure. - Neither sensitivity nor specificity uses both true positives and true negatives in the denominator.

Sensitivity/Specificity US Medical PG Question 4: A 25-year-old man comes to the physician for severe back pain. He describes the pain as shooting and stabbing. On a 10-point scale, he rates the pain as a 9 to 10. The pain started after he lifted a heavy box at work; he works at a supermarket and recently switched from being a cashier to a storekeeper. The patient appears to be in severe distress. Vital signs are within normal limits. On physical examination, the spine is nontender without paravertebral muscle spasms. Range of motion is normal. A straight-leg raise test is negative. After the physical examination has been completed, the patient asks for a letter to his employer attesting to his inability to work as a storekeeper. Which of the following is the most appropriate response?

• A. “Yes. Since work may worsen your condition, I would prefer that you stay home a few days. I will write a letter to your employer to explain the situation.”
• B. You say you are in severe pain. However, the physical examination findings do not suggest a physical problem that can be addressed with medications or surgery. I'd like to meet on a regular basis to see how you're doing.
• C. I understand that you are uncomfortable, but the findings do not match the severity of your symptoms. Let's talk about the recent changes at your job. (Correct Answer)
• D. The physical exam findings do not match your symptoms, which suggests a psychological problem. I would be happy to refer you to a mental health professional.
• E. The physical exam findings suggest a psychological rather than a physical problem. But there is a good chance that we can address it with cognitive-behavioral therapy.

Sensitivity/Specificity Explanation: ***"I understand that you are uncomfortable, but the findings do not match the severity of your symptoms. Let's talk about the recent changes at your job."*** - This response acknowledges the patient's reported discomfort while gently highlighting the **discrepancy between symptoms and objective findings**, which is crucial in cases of suspected **somatoform or functional pain**. - It also opens communication about potential **psychosocial stressors** related to his job change, which could be contributing to his symptoms, without dismissing his pain or making a premature diagnosis. *"You say you are in severe pain. However, the physical examination findings do not suggest a physical problem that can be addressed with medications or surgery. I'd like to meet on a regular basis to see how you're doing."* - While this option correctly identifies the lack of physical findings, it can be perceived as dismissive of the patient's pain, potentially damaging the **physician-patient relationship**. - Suggesting regular meetings without a clear plan for addressing his immediate concerns or exploring underlying issues might not be the most effective initial approach. *“Yes. Since work may worsen your condition, I would prefer that you stay home a few days. I will write a letter to your employer to explain the situation.”* - This response would **validate the patient's claim of severe pain** without objective evidence, potentially reinforcing illness behavior and avoiding addressing the underlying issue. - Providing a doctor's note for inability to work without a clear diagnostic basis or understanding of the pain's origin is **medically inappropriate** and could set a precedent for future such requests. *"The physical exam findings do not match your symptoms, which suggests a psychological problem. I would be happy to refer you to a mental health professional."* - Directly labeling the problem as "psychological" can be **stigmatizing and alienating** to the patient, leading to distrust and resistance to care. - While a psychological component might be present, immediately referring to mental health without further exploration of the patient's situation or current stressors is premature and lacks empathy. *"The physical exam findings suggest a psychological rather than a physical problem. But there is a good chance that we can address it with cognitive-behavioral therapy."* - Similar to the previous option, explicitly stating a "psychological problem" can be **stigmatizing**. - Jumping directly to recommending **cognitive-behavioral therapy (CBT)** without a comprehensive discussion and patient buy-in is premature and may lead to non-compliance.

Sensitivity/Specificity US Medical PG Question 5: The World Health Organization suggests the use of a new rapid diagnostic test for the diagnosis of malaria in resource-limited settings. The new test has a sensitivity of 70% and a specificity of 90% compared to the gold standard test (blood smear). The validity of the new test is evaluated at a satellite health center by testing 200 patients with a positive blood smear and 150 patients with a negative blood smear. How many of the tested individuals are expected to have a false negative result?

• A. 60 (Correct Answer)
• B. 15
• C. 135
• D. 155
• E. 195

Sensitivity/Specificity Explanation: ***Correct Option: 60*** - **False negatives** occur in individuals who have the disease but test negative. This is directly related to the test's **sensitivity**. - Given a sensitivity of 70%, 30% of actual positive cases (100% - 70%) will be missed. With 200 patients having a positive blood smear (meaning they have malaria), 30% of 200 is 0.30 × 200 = **60**. *Incorrect Option: 15* - This number represents the expected number of **false positives** (150 patients without disease × 10% false positive rate = 15). - However, the question asks for **false negatives**, not false positives. *Incorrect Option: 135* - This value represents the number of **true negatives** (150 patients without malaria × 90% specificity = 135). - It does not represent false negative results. *Incorrect Option: 155* - This appears to be a distractor number that doesn't correspond to any standard diagnostic test calculation in this scenario. - It does not represent false negatives or any meaningful combination of the given parameters. *Incorrect Option: 195* - This number might be derived from incorrectly applying formulas or miscalculating the relationship between sensitivity and false negatives. - It does not represent the correct calculation for false negatives.

Sensitivity/Specificity Flashcard 1 of 5

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Sensitivity/Specificity Flashcard 3 of 5

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Sensitivity/Specificity Flashcard 5 of 5

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