Ultrasound against a HIDA scan for suspected cholecystitis, appraised before ordering, with accuracy figures cited, ratios and post-test probabilities computed, and the decision each result would drive written out. Searches like "nu 552 unit 4 assignment example", "nu552 unit 4 sample" and "nu552 unit 4 example" land here.
What Would Each Result Change? Appraising Ultrasound and Scintigraphy for Suspected Acute Cholecystitis
[Student Name]
Purdue University Global
NU552: Advanced Health Assessment and Diagnostic Reasoning
Unit 4 Assignment
[Instructor Name]
[Date]
The patient is a composite built for the assignment; no real patient is described.
The Clinical Question
Ms. [K.], 46, an office manager, developed constant right upper quadrant pain about an hour after a fried dinner. Six hours later it has not eased. She has vomited twice. Her temperature is [37.9] C, and she winces and stops breathing in when the examiner presses under the right costal margin as she inhales, a positive Murphy sign. Her white cell count is [12,400] per microliter.
The question is whether she has acute cholecystitis, inflammation of the gallbladder, usually from a stone lodged in its neck, or biliary colic, pain from a stone that obstructs briefly and passes, or something else. The distinction matters because acute cholecystitis usually needs admission, antibiotics and early removal of the gallbladder, while biliary colic can be managed as an outpatient.
The Pretest Estimate
No single finding in the history or examination is enough to diagnose or exclude acute cholecystitis (Trowbridge et al., 2003). Taken together, persistent pain beyond four to six hours, a low-grade fever, a positive Murphy sign and a raised white count make it a serious possibility. The Tokyo Guidelines, which combine local signs, systemic signs and imaging into a diagnostic framework, would classify her as suspected but not yet definite, because imaging has not been done (Yokoe et al., 2018). The pretest probability is estimated at [40] percent.
What Each Result Would Change
Before any test is ordered, the thresholds are set:
1. Above about [70] percent: treat as acute cholecystitis, admit and ask for a surgical consultation.
2. Below about [5] percent: stop looking for cholecystitis and pursue other explanations.
3. Between those levels: the diagnosis is neither made nor excluded, and further testing is justified.
A test is worth ordering only if at least one of its possible results would move the probability across a threshold and change what happens next.
The Accuracy Figures
The figures come from a meta-analysis of imaging in acute cholecystitis (Kiewiet et al., 2012):
1. Ultrasound: sensitivity [0.81], specificity [0.83].
2. Hepatobiliary scintigraphy: sensitivity [0.96], specificity [0.90].
Likelihood ratios were calculated as sensitivity divided by one minus specificity for a positive result, and one minus sensitivity divided by specificity for a negative result.
1. Ultrasound: positive ratio 0.81 divided by 0.17, or 4.8; negative ratio 0.19 divided by 0.83, or 0.23.
2. Scintigraphy: positive ratio 0.96 divided by 0.10, or 9.6; negative ratio 0.04 divided by 0.90, or 0.044.
The Two-by-Two Table
The table uses [100] hypothetical patients like Ms. [K.], at a prevalence of 40 percent, because counts are easier to follow than conditional percentages. Forty have acute cholecystitis and sixty do not.
Ultrasound:
1. True positives: 81 percent of 40, about 32.
2. False negatives: about 8.
3. False positives: 17 percent of 60, about 10.
4. True negatives: about 50.
Of 42 positive results, 32 are true, a positive predictive value of 76 percent. Of 58 negative results, 50 are true, a negative predictive value of 86 percent.
Scintigraphy:
1. True positives: 96 percent of 40, about 38.
2. False negatives: about 2.
3. False positives: 10 percent of 60, about 6.
4. True negatives: about 54.
Of 44 positive results, 38 are true, a positive predictive value of 86 percent. Of 56 negative results, 54 are true, a negative predictive value of 96 percent.
Post-Test Probabilities
The same results follow from the likelihood ratios. A pretest probability of 40 percent is odds of 0.67.
1. Ultrasound positive: 0.67 times 4.8 gives odds of 3.2, a probability of 76 percent.
2. Ultrasound negative: 0.67 times 0.23 gives odds of 0.15, a probability of 13 percent.
3. Scintigraphy positive: 0.67 times 9.6 gives odds of 6.4, a probability of 87 percent.
4. Scintigraphy negative: 0.67 times 0.044 gives odds of 0.03, a probability of 3 percent.
Measured against the thresholds, a positive ultrasound crosses the treatment threshold, and a negative scintigram crosses the stopping threshold. A negative ultrasound, at 13 percent, crosses neither.
Accuracy Is Not the Whole Question
By the numbers alone, scintigraphy is the better test. It is more sensitive and more specific, and its negative result nearly excludes the diagnosis. But accuracy is not the only property that decides which test to order first.
Ultrasound is quick and widely available at the bedside or in the radiology department, often within the hour. It involves no ionizing radiation. And it answers more than one question at once: it shows gallstones, gallbladder wall thickening and fluid around the gallbladder, and it also images the bile ducts, the liver and part of the pancreas. If the common bile duct is dilated, suggesting a stone in the duct or cholangitis, or if the pain turns out to come from the liver or pancreas, ultrasound may reveal it. Scintigraphy answers only whether the cystic duct is open. It takes longer, often several hours, requires a radioactive tracer and is not available everywhere, especially at night.
Is 13 Percent Low Enough to Stop?
If the ultrasound is negative, Ms. [K.]'s probability falls to about 13 percent. That is not low enough to stop by the threshold set above. Whether to pursue scintigraphy then depends on how she looks. If her pain settles, her temperature normalizes and her Murphy sign disappears over the next few hours, a residual probability of 13 percent may be acceptable, with outpatient follow-up and clear instructions to return. If her pain persists, her fever rises or her white count climbs, the probability in practice is higher than the calculation suggests, and scintigraphy is justified to settle the question.
Limits of the Figures
The calculations rest on three assumptions that deserve scrutiny. First, pooled sensitivity and specificity come from studies whose patients may differ from Ms. [K.]. Studies that enroll sicker patients tend to report higher sensitivity, and studies in which the reference standard was applied only to some patients can inflate accuracy. Second, ultrasound is operator-dependent: an experienced sonographer who elicits a sonographic Murphy sign and measures wall thickness carefully will perform closer to the upper end of the reported range than an inexperienced one at night. Third, the pretest estimate of 40 percent is a judgment, not a measurement. If it were 25 percent instead, a negative ultrasound would leave about 7 percent, and a positive one about 62 percent, below the treatment threshold. Every post-test figure should therefore be read as approximate, and the thresholds as guides rather than rules.
Recommendation
The recommendation is sequential. Order ultrasound first, because it is fast, safe and able to answer several questions. If it is positive, treat as acute cholecystitis. If it is negative or equivocal and she remains unwell, order scintigraphy, whose negative result can exclude the diagnosis and whose positive result can confirm it. Each step is tied to the result that would trigger it, and the thresholds were set before the first test was ordered.
References
Kiewiet, J. J. S., Leeuwenburgh, M. M. N., Bipat, S., Bossuyt, P. M. M., Stoker, J., & Boermeester, M. A. (2012). A systematic review and meta-analysis of diagnostic performance of imaging in acute cholecystitis. Radiology, 264(3), 708-720. https://doi.org/10.1148/radiol.12111561
Trowbridge, R. L., Rutkowski, N. K., & Shojania, K. G. (2003). Does this patient have acute cholecystitis? JAMA, 289(1), 80-86. https://doi.org/10.1001/jama.289.1.80
Yokoe, M., Hata, J., Takada, T., Strasberg, S. M., Asbun, H. J., Wakabayashi, G., Kozaka, K., Endo, I., Deziel, D. J., Miura, F., Okamoto, K., Hwang, T.-L., Huang, W. S.-W., Ker, C.-G., Chen, M.-F., Han, H.-S., Yoon, Y.-S., Choi, I.-S., Yoon, D.-S., ... Yamamoto, M. (2018). Tokyo Guidelines 2018: Diagnostic criteria and severity grading of acute cholecystitis. Journal of Hepato-Biliary-Pancreatic Sciences, 25(1), 41-54. https://doi.org/10.1002/jhbp.515
How this NU552 Unit 4 example is structured
One question organizes the appraisal: what would each result change? Every figure serves it. Accuracy values are bracketed and sourced to a meta-analysis, and each derived number is shown with its working. The table is built for 100 hypothetical patients because counts are easier to follow than conditional percentages. The paper then separates statistical performance from practical fit: scintigraphy is more accurate, but ultrasound is faster, involves no radiation and also images the bile ducts and liver, widening what one study can answer. A threshold paragraph asks whether a 13 percent residual probability is low enough to stop, and answers that it depends on how the patient looks. The recommendation is sequential, ultrasound first and scintigraphy reserved for the equivocal case, each step tied to the result that would trigger it.
Get an NU552 Unit 4 example written to your instructions
Name the tests your NU552 Unit 4 prompt compares, or just the clinical question when no test is named, and include the rubric and any accuracy figures provided. Within 24-48h, and free the first time, an appraisal comes back to your instructions with every ratio and post-test figure calculated in plain view and tied to a decision. The paper above is an original model document written by our desk, not a submitted student paper and not an official Purdue University Global document.
NU552 Unit 4 questions, answered
What if my prompt names a different test?
The method transfers to any test: state the question, set a pretest probability with a reason, calculate ratios and post-test figures from cited accuracy values, and write down what each result would change. The sample uses imaging for gallbladder disease, but a laboratory marker or a screening test would be appraised in exactly the same way.
Is a two-by-two table required?
Not always, but it helps readers and graders check the arithmetic. Building it for a round number of hypothetical patients makes false positives and false negatives concrete. If your rubric asks only for likelihood ratios, the table can move to an appendix, and either arrangement is available to suit your instructor.
How is the pretest probability chosen?
From the presentation, the setting and, where available, a published estimate or clinical rule. The sample justifies its figure from fever, a positive Murphy sign and the meal history, and brackets it to show it is an estimate. Stating the reason matters more than precision, because every later number depends on it.