Becoming an expert consumer of Medical Statistics



FPIN Journal Club

DIAGNOSTIC STUDY

SPEAKER NOTES

1. What question did the study attempt to answer?

Patients – 286 patients with uncomplicated essential HTN. Just over half (53%) women, avg age 64, avg BMI 29.4, avg number of BP medications 2.4.

Intervention – home BP monitoring

Comparison – ambulatory BP monitoring

Outcome – ability to correctly predict elevated BP

Did the study address an appropriate and clearly focused question Yes No

2. Determining Relevance:

a. Is the diagnostic test feasible and common to your practice? Yes No

b. Is the proportion of patients with the target illness comparable

to the patient group seen by family physicians? Yes No

c. Did the authors study a clinically meaningful Yes No

and/or a patient oriented outcome?

3. Determining Validity:

a. What test is being evaluated

Home blood pressure monitoring—2 readings taken one minute apart, three times daily (morning, noon, evening)

b. What is the reference standard with which the test being evaluated is compared?

Ambulatory blood pressure monitoring—24 hours of readings q 30 min during daytime and q 60 min overnight

c. The test and reference standard are measured

independently (blind) of each other? Yes No Unclear

Not clear, but not likely to bias objective BP measurements.

d. Did the patient sample include an appropriate

spectrum of patients to whom the diagnostic test will be

applied in clinical practice? Yes No Unclear

e. Patients for testing are selected either as a consecutive

series or randomly, from a clearly defined study Yes No Unclear

f. Results are reported for all patients that are entered

into the study Yes No Unclear

No info given on N for whom complete info reported

4. What are the results?

a. What is the estimated sensitivity of the test being evaluated? (state 95% CI)

Sensitivity = proportion of results in patients with the disease that are correctly identified by the new test

The sensitivity of ≥3 elevations out of last 10 home readings for mean 24-hour ABP systolic readings ≥130 mm Hg was 62% and for 24-hour ABP daytime systolic readings ≥135 mm Hg was 65%.

b. What is the estimated specificity of the test being evaluated (state 95% CI)

Specificity = proportion of results in patients without the disease that are correctly identified by the new test

Specificity of ≥3 elevations out of last 10 home readings for mean 24-hour ABP systolic readings ≥130 mm Hg was 80%, and for 24-hour ABP daytime systolic readings ≥135 mm Hg was 77%.

c. What are the likelihood ratios for the test being evaluated?

LR+ = sens / (1-spec)

- LR+ > 10 indicates a large change in likelihood, < 2 indicates no change in likelihood

LR+ for detecting ABP ≥130 = 0.62/(1-0.80) = 3.1

LR+ for detecting ABP ≥135 = 0.65/(1-0.77) = 2.8

LR− = (1-sens) / spec

- LR− < 0.1 indicates a large change in likelihood, > 0.5 indicates no change in likelihood

LR- for detecting ABP ≥130 = (1-0.62)/0.8 = 0.48

LR- for detecting ABP ≥135 = (1-0.65)/0.77 = 0.45

Teaching Points: Likelihood ratios

These ratios tell you how much the odds of a disease increase when a test is positive (LR+) or how much the odds of a disease decrease when a test is negative (LR-). Likelihood ratios are independent of prevalence. They can be calculated from the sensitivity and specificity (formulas above).

Positive likelihood ratio -– How likely is the disease present if a test is positive

• It should be > 1, the higher the better

• LR + > 10 is an ideal test

• LR+ < 2 is no change

Negative likelihood ratio – How likely is the disease not present with a negative test

• It should be < 1, the lower the better

• LR- < 0.1 is an ideal test

• LR- > 0.5 is no change

5. Applying the evidence

a. Will the results help me in caring for my patients Yes No

b. If the findings are valid and relevant, will this change

your current practice? Yes No

Probably—it depends on how often provider was recommending home monitoring, and availability/acceptability of ambulatory monitoring

c. Is the change in practice something that can be done in

a medical care setting of a family physician? Yes No

d. Can the results be implemented? Yes No

e. Are there any barrier to immediate implementation? Yes No

Cost of appropriate cuff as well as need for patient education to collect valid readings at home

f. How was this study funded? Government grant funded, National Health and Medical Research Council of Australia

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