Dose Effect
Explained
So far we have only considered that persons could be either exposed or not, but exposure can frequently be quantified and a gradient established. Examples may include the number of glasses (or volume) of water consumed per day, the number of pizza slices eaten, or the degree of completion of a vaccination course etc. The risk of illness can then be calculated according to amount drunk, eaten or completed.
| Amount drunk | Exposed | Cases | Risk (%) | RR |
|---|---|---|---|---|
| None | 100 | 5 | 5 | ref |
| 1 glass | 100 | 11 | 11 | 2.2 |
| 2 glasses | 100 | 16 | 16 | 3.2 |
| 3 glasses | 100 | 22 | 22 | 4.4 |
| ≥4 glasses | 100 | 28 | 28 | 5.7 |
| 500 | 82 | 82 | c |
The Χ2 test for trend across these strata = 23.24, p≤0.001 - indicating that there is a trend across the exposure strata.
If instead a case control study had been conducted (note the similarity between the OR and the RR above):
| Amount drunk | Exposed | Cases | OR |
|---|---|---|---|
| None | 5 | 30 | ref |
| 1 glass | 11 | 30 | 2.2 |
| 2 glasses | 16 | 30 | 3.2 |
| 3 glasses | 22 | 30 | 4.4 |
| ≥4 glasses | 28 | 30 | 5.6 |
| 82 | 150 |
For which the Χ2 test for trend = 12.77, p≤0.001
In the above examples, the risk of disease occurring increases with the amount of water consumed (i.e. with the dose of water). This is frequently called a dose response or a dose effect relationship.
To compute the risk ratio or the odds ratio the same reference level of exposure is used. Here the non exposed group is used as a reference. Alternatively the group with the lowest incidence could be used.
Dose response is particularly interesting when in an outbreak (often in water borne or a food borne outbreak) everybody is exposed to the same factor but the level of exposure differs among individuals. Choosing the lowest category of exposure as a reference still allows identification of a food item or a beverage as the vehicle for the outbreak. To test that the trend observed in risk or odds ratio differs from no trend, we can use a chi square for trend [1], [2], [3]. Dose response is also an important criterion to consider when providing evidence for causality proposed by Bradford Hill [4].
References
- ↑ J.J.Schlesselman. Case-control studies: design, conduct, analysis. New York: Oxford University Press; 1982.
- ↑ B.R.Kirkwood, J.A.C.Sterne. Medical Statistics. Second ed. Massachusetts: Blackwell Science Ltd; 2003.
- ↑ K.J.Rothman, S.Greenland, T.L.Lash. Modern Epidemiology. Third ed. Philadelphia, USA: Lipincott Williams and Wilkins; 2008.
- ↑ A.Bradford Hill. The Environment and Disease: Association or Causation? Proceedings of the Royal Society of Medicine 1965;58:295-300.