Blinding (or Masking) in clinical trials means that either participants, investigators, or the staff study like the outcome assessors or research assistants, or analysts, do not know which group takes the intervention or the placebo.
Types of Blinding in Clinical Trials
Usually, we classify the blinding to:
1. Single-blind: where the participants do not know in which group they are.
2. Double-blind: where the participants and the investigators do not know who take the intervention or the placebo.
3. Triple-blind: it depends on the trial who is blinded, like outcome ascertainment, a safety monitoring group, an analyst, or lab test performer, … etc.
It is better to specify in the protocol or in the manuscript who is blinded rather than describe the blinding as single, double, or triple.
Here are some kinds of trials where it is hard to do blinding:
1. If the intervention was diet or exercise.
2. If it was an educational trial.
3. If the trial was conducting surgery, so it will be unethical to do a sham surgery on the control group.
4. Sometimes it may be difficult to prepare a placebo that looks like an intervention, or the intervention may have taste, smell, or feel a little different.
5. If the intervention causes side effects while the placebo does not.
The importance of blinding in RCTs refers to:
- It can minimize the difference between the two groups throughout the whole trial and during the follow-up, while randomization can minimize the difference at baseline only.
- Minimize the co-intervention effects (unintended intervention) between the two groups.
- Minimize the biased outcome ascertainment when all participants report symptoms in the same way in two groups.
- Minimize the biased measurement of the outcome especially if there is a subjective outcome (soft outcome) like chest pain, satisfaction, or disability.
If we think that it has been any unblinding in trial, we can measure the degree of the unblinding, by asking the participants and study staff to guess the treatment of each group, the proportion should be the same in each group, about 50% in each group, but if it was greater than that, it can be considered as an unblinding trial.
At last, if we can’t do blinding, we have at a minimum blinded who is measuring, adjudicating, or ascertaining the outcomes, and that will help to reduce bias in the two groups and ensure a reliable result.
Read also: Clinical Trial Feasibility Study
Resource Person: Nesreen Alshamaly
