In addition to large effect, GRADE uses another of the causal criteria from Bradford Hill to upgrade the evidence. The second domain for upgrading recognizes that larger exposures should generally lead to increases in occurrence of the outcome. This is known as a dose-response. As the dose, or exposure, increases so do more cases of the disease or condition of interest.
Yesterday's example of the association of smoking with development of lung cancer provides another example of this effect. Not only is the observed effect of smoking on lung cancer strong, but also includes a dose-response with increasing incidence of lung cancer as people smoke more cigarettes per day.
This provides an opportunity to explain how upgrading works. Since the effect of smoking is so strong, (remember from yesterday that current data shows a 25x increased risk of lung cancer in smokers compared to non-smokers) the evidence would be upgraded two levels from the starting of low confidence in the evidence. This increases the rating to high confidence. When you add the impact of a dose response, you cannot raise the evidence rating above high. Still, you would note the dose response on the evidence profile. We'll take look at a couple of evidence profiles later in the series.
So there are two rather straight-forward reasons to upgrade the evidence rating for evidence derived from observational studies:
- Upgrade for strong effect
- Upgrade for dose - response
Remember the observational studies must be well designed and well conducted to be eligible for upgrading.
Tomorrow I'll finish the section on upgrading with the final domain.
As always, for more detail check out the the GRADE Handbook
TheEvidenceDoc November 29, 2017