What a negative rapid strep result means for a child, reasoned from a 500-child table built on Cochrane pooled accuracy 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.
About 6 in 100 Negatives Still Have Strep: A Diagnostic Test Analysis of the Rapid Antigen Test in a Febrile Nine-Year-Old
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Purdue University Global
NU505: Clinical Epidemiology and Population Health Promotion
Unit 5 Assignment
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[Date]
The clinic, the child and the 500-child table are composite; the table is built from the pooled accuracy reported in a Cochrane review, and the guidance cited is published.
The Question
A nine-year-old arrives at a composite pediatric urgent care with fever of 38.9 degrees Celsius, sore throat, tender anterior cervical nodes and exudate on both tonsils, without cough. The rapid antigen detection test is negative. What does that negative mean, and should a throat culture be sent?
The Table
A Cochrane review of rapid antigen tests in children with pharyngitis pooled a sensitivity of 85.6 percent and a specificity of 95.4 percent against throat culture, in studies with a median prevalence of group A strep near 30 percent (Cohen et al., 2016). Applying those values to 500 children at a clinic where 30 percent have culture-confirmed strep gives this table.
1. Culture positive, 150 children: rapid test positive in 128, true positives; negative in 22, false negatives.
2. Culture negative, 350 children: rapid test positive in 16, false positives; negative in 334, true negatives.
3. Totals: 144 positive tests; 356 negative tests.
Properties of the Test
Sensitivity: 128 of 150, 85.3 percent. Specificity: 334 of 350, 95.4 percent. These describe the test in the population it was validated on. They are often treated as fixed, but they can shift with how sick the children are; tests tend to detect heavier bacterial loads more easily, and the review found substantial variation in sensitivity between studies.
Values That Belong to This Setting
Positive predictive value: 128 of 144, 88.9 percent. Negative predictive value: 334 of 356, 93.8 percent, so 22 of every 356 negative results, about 6 in 100, come from children who have strep. These predictive values belong to a clinic with 30 percent prevalence, not to the test; the same test would give different values in a different clinic.
Likelihood Ratios as the Bridge
Positive likelihood ratio: sensitivity divided by one minus specificity, 0.853 divided by 0.046, about 18.7. Negative likelihood ratio: one minus sensitivity divided by specificity, 0.147 divided by 0.954, about 0.15. The ratios let the estimate move from any starting probability. Convert the pre-test probability to odds, multiply by the ratio, and convert back.
This Child
The child is not an average member of the clinic population. Fever, exudate, tender nodes, no cough and an age between 3 and 14 together raise the pre-test probability well above the clinic's 30 percent. Taking a pre-test probability of 50 percent for this presentation, the pre-test odds are 1. Multiplying by 0.15 gives post-test odds of about 0.15 and a post-test probability of about 13 percent. At the clinic's average of 30 percent, the same negative leaves about 6 percent.
Either way, a negative rapid test does not clear this child. Roughly one child in eight with this presentation and a negative test would still have strep.
How Much to Trust the Pooled Values
The table rests on pooled accuracy, and the review behind it judged many included studies at high risk of bias, mostly in how patients were selected and in the reference standard (Cohen et al., 2016). Two biases matter most here. Spectrum bias arises when validation studies enroll children who are sicker, or less sick, than the children a clinic actually sees, which can inflate or deflate sensitivity. Verification bias arises when only some children receive the reference culture, often those with positive tests, which tends to overstate sensitivity. The review restricted its main estimate to studies in which all children had both tests, which reduces the second problem. The practical lesson is that the 6 in 100 figure is an estimate with its own uncertainty; a clinic that tracks its own backup cultures can check it.
The Adult Comparison
In an adult clinic where perhaps 10 percent of patients with sore throat have group A strep, the same negative result moves the probability from 10 percent to under 2 percent: pre-test odds of 0.11, times 0.15, gives post-test odds of about 0.017. The test is identical; the setting is not.
The Decision
The difference in settings is why guidance differs by age. National guidance for group A strep pharyngitis calls for backing up a negative rapid test with a throat culture in children and adolescents, but not routinely in adults (Shulman et al., 2012). The reasons are the ones this analysis shows, plus one more. In children, prevalence is higher, so more negatives are false. And untreated streptococcal pharyngitis in school-age children carries a small risk of acute rheumatic fever, a complication that is rare in adults. For this nine-year-old, a backup throat culture should be sent, the family told that results take a day or two, and antibiotics started if the culture is positive.
What the Analysis Shows
Three layers have to stay separate: the test's properties, the setting's prevalence and the individual child's pre-test probability. Sensitivity and specificity belong to the test. Predictive values belong to the test as used in one clinic. Likelihood ratios let a clinician apply the test to one child. Celentano and Szklo (2019) put the same point in general terms: the value of a test result depends on the population in which it is used. For this child, a negative rapid test is useful information and not a diagnosis.
References
Celentano, D. D., & Szklo, M. (2019). Gordis epidemiology (6th ed.). Elsevier.
Cohen, J. F., Bertille, N., Cohen, R., & Chalumeau, M. (2016). Rapid antigen detection test for group A streptococcus in children with pharyngitis. Cochrane Database of Systematic Reviews, 2016(7), CD010502. https://doi.org/10.1002/14651858.CD010502.pub2
Shulman, S. T., Bisno, A. L., Clegg, H. W., Gerber, M. A., Kaplan, E. L., Lee, G., Martin, J. M., & Van Beneden, C. (2012). Clinical practice guideline for the diagnosis and management of group A streptococcal pharyngitis: 2012 update by the Infectious Diseases Society of America. Clinical Infectious Diseases, 55(10), e86-e102. https://doi.org/10.1093/cid/cis629
How this 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.
Get an 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. 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.
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.