A randomised controlled trial examining a treatment for septic shock reports the following results:

"At 90 days after randomization, 27.9% patients who had been assigned to receive the treatment had died, as had 28.8% who had been assigned to receive placebo (odds ratio 0.95; 95% confidence interval [Cl], 0.82 to 1.10; P value= 0.50)."

 
a) Explain the meaning of the underlined terms. Interpret the result of the trial.

(40% marks)
 

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College answer

Odds ratio: The odds of a patient in the treatment group dying within 90 days divided by the odds of patients in the placebo group dying within 90 days.

95% confidence interval: The range of values which is 95% certain to contain the population parameter of interest (in this case, Odds Ratio)

P Value: The probability of obtaining the observed, or more extreme results, assuming the null hypothesis is true.    (3 marks)
 

Discussion

In case it matters to anybody, in this SAQ the examiners are using the findings of the ADRENAL trial (Venkatesh et al, 2018)

Odds ratio:

  • The Odds Ratio represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure.
  • An OR =1 suggests there is no association.
  • If the CI for an OR includes 1, then the OR is not significant (i.e. there might not be an association)

Confidence interval:

  • The range of values within which the "actual" result is found.
  • A CI of 95% means that if a trial was repeated an infinite number of times, 95% of the results would fall within this range of values.
  • The CI gives an indication of the precision of the sample mean as an estimate of the "true" population mean
  • A wide CI can be caused by small samples or by a large variance within a sample.

  p-value:

  • The probability of the observed result arising by chance
  • The p-value is the chance of getting the reported study result (or one even more extreme) when the null hypothesis is actually true.

Interpretation of results:

  • The OR is so close to 1 that there is probably no association if the results were significant, i.e. there is no treatment effect detected here. 

References