Unit quiz: Time and course: prevalence, prognosis, relapse and the shape of response

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Question 1

A fictional community mental health team reports that 34% of its current caseload meets criteria for a psychotic disorder, against a pooled community incidence of about 27 per 100,000 person-years. A manager concludes that the local area has an epidemic. Using lesson 81, what is the most important correction?A clinic caseload is a prevalent pool, not a population: it accumulates people whose illness is long, who remain in contact, and who were referred, so a high proportion in the clinic is compatible with ordinary incidence and long duration. The comparable quantity is incidence in a defined population, not caseload composition.The manager is right to be concerned, because a caseload proportion that high can only arise from an incidence rate several hundred times the pooled estimate, and the team should audit its diagnostic practice before anything else.

Question 2

A fictional trust proposes to screen all new prisoners with a 12-item instrument for suicide risk. Validation in the development sample gives sensitivity 0.70 and specificity 0.75; the annual incidence of the outcome in this population is about 2%. An executive asks what proportion of those flagged will actually go on to the outcome.About 70 per cent, because sensitivity is the proportion of true cases the instrument detects, so seven in ten flagged people will be true cases.Roughly 5 to 6 per cent. With a 2% base rate, of 1,000 people about 20 have the outcome and 14 of those are flagged (sensitivity 0.70), while about 245 of the 980 without it are also flagged (specificity 0.75), giving about 259 flags and a positive predictive value near 5.4%.

Question 3

A fictional trainee summarises a maintenance trial: relapse 27% on continued antipsychotic against 64% on placebo over one year, and tells a patient that continuing treatment will cut her risk of relapse by about 60%. The patient asks what that means for her.The relative reduction is real but is not her personal probability; the absolute difference is 37 percentage points over one year, so about three people must be treated for one year to prevent one relapse, and the same trial reports harms (weight gain 10% versus 6%, movement disorders 16% versus 9%) that belong in the same conversation.The figure of 60% is the correct way to express it, because risk ratios are the standard currency of trial reporting and converting to absolute terms would overstate certainty.

Question 4

A fictional clinic protocol states that patients who have not responded at the minimum licensed dose should have the dose doubled before any switch is considered, on the grounds that higher doses work better. A trainee asks what the dose-response evidence supports.Dose-response is monotonic for all antidepressants, so doubling the dose is the evidence-based next step and switching should be reserved for people who fail two adequate trials.For SSRIs and mirtazapine, flexible titration above the minimum licensed dose has not been shown to add efficacy, tolerability or acceptability, and fixed-dose dose-response analyses place the best balance of efficacy and dropout at the lower part of the licensed range; the exception in that literature is venlafaxine between 75 and 150 mg.

Question 5

A fictional commentary notes that clinic records show far lower rates of anxiety disorder among people over 70 than among people in their thirties, and concludes that anxiety burns out with age. Using lesson 85, what is the strongest objection?A cross-sectional age difference cannot be assigned to ageing: it confounds age, cohort and period effects, and clinic records add selection, since older people who attend may differ from their birth cohort and those with long-standing difficulties may have left services or died.The finding is sound because the sample is large, and the only real limitation is that older people under-report symptoms, which would if anything make the true age decline steeper.

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