What a negative rapid strep result means for a child, reasoned from a composite 500-child validation table through to the backup culture decision, in NU505's Unit 5 analysis. Searches like "nu 505 unit 5 assignment example", "nu505 unit 5 sample" and "nu505 unit 5 example" land here.
What a finished NU505 Unit 5 diagnostic test analysis looks like
Three pages, organized around one table and one decision. The table sets rapid test results against throat culture for 500 children aged 5 to 15 at a composite pediatric urgent care, where 30 percent had culture-confirmed strep: 129 true positives, 21 false negatives, 21 false positives and 329 true negatives. Derived first are sensitivity, 86 percent, and specificity, 94 percent, then predictive values of 86.0 for a positive and 94.0 for a negative. A likelihood ratio line follows, 14.3 for a positive and 0.15 for a negative. The next section places the nine-year-old in the table and reads the negative result as a post-test probability near 6 percent. Set beside it, an adult population with lower prevalence shows why the same negative clears the diagnosis more convincingly there. The decision section closes the paper.
How a NU505 Unit 5 example is structured
From table to test properties to setting-dependent values to one patient, the analysis keeps its three layers separate. First come sensitivity and specificity, framed as properties of the test in this validation population, with a note that they can shift with how sick the children were. Predictive values follow and are labeled as belonging to a 30 percent prevalence, not to the test. The likelihood ratios bridge the two, since they allow the estimate to move from any starting probability. The patient paragraph then applies the numbers, and the analysis explains why roughly 6 in 100 remaining cases matters in children: untreated streptococcal pharyngitis carries a small risk of rheumatic fever, which is why pediatric guidance commonly calls for backup culture after a negative rapid test. The adult comparison shows the same test supporting a different action when prevalence and the complication risk are lower.
Throat culture as the reference
Every child had both tests, with culture read without knowledge of the rapid result. The analysis states that design point first, since verification of only some children would distort every figure below.
Properties of the test
At 86 percent sensitivity, 21 of 150 infected children were missed. At 94 percent specificity, 21 of 350 without strep were flagged. Both are tied to this validation group.
Values that belong to the setting
At 30 percent prevalence, a positive is right 86 times in 100 and a negative 94. The analysis marks both as local, because another clinic's case mix would change them.
One child after a negative result
Starting from the clinic's 30 percent, the negative result lowers the probability to about 6 percent. Low, but not low enough to dismiss in a child, and the analysis explains the rheumatic fever reasoning.
Backup culture, and when it is skipped
Children with a negative rapid test receive a throat culture before antibiotics are ruled out. In adults, lower prevalence and very low rheumatic fever risk generally make backup culture unnecessary.
Where marks go in NU505 Unit 5
Diagnostic analyses in this course are typically marked in sequence: the table, the derived measures, their interpretation in context, and the clinical decision they support. Construction marks depend on placing the reference standard correctly and totaling rows and columns. Derivation errors cluster around predictive values computed across the wrong margin. Interpretation is the heaviest row in many rubrics: calling 94 percent negative predictive value reassuring without saying what the other 6 percent means for a child misses the clinical point. Decision credit requires a named action tied to the numbers and to guidance, not a general statement that tests are imperfect. Treating sensitivity as the probability that a positive child has strep draws deductions too, as does ignoring spectrum effects when the validation population differs from the patient being assessed.
Get a NU505 Unit 5 example written to your instructions
Diagnostic prompts in Unit 5 usually hand over a table or a published accuracy study. Send yours along with the patient scenario, if there is one, and your rubric. The free first custom sample follows those instructions and returns in 24-48h, reasoning from the cells to the decision a clinician would face.
NU505 Unit 5 questions, answered
Why do children need a backup culture after a negative rapid strep test but adults usually do not?
Two things differ: how common strep is and how serious a missed case could be. Strep causes a larger share of sore throats in school-aged children, and untreated infection carries a small risk of rheumatic fever that is highest in that age group. In adults, both prevalence and complication risk are lower, so a negative rapid test is generally accepted.
Are likelihood ratios necessary in this analysis?
They are useful because they let the result update any starting probability, not just the prevalence in the validation study. A negative ratio of 0.15 applied to a child with a higher or lower pretest estimate gives a different post-test figure. Some sections require them; others accept predictive values alone. The example includes them in one line without letting them dominate.
What is spectrum bias, and does it apply here?
Spectrum bias occurs when a test's accuracy varies with the mix of patients studied. Rapid strep tests tend to be more sensitive in children with more classic signs. If the validation group was sicker than the patients a clinic usually sees, sensitivity in practice may be lower. The example notes this as a limit on transferring its figures to milder presentations.