As in the topic. I saw many critical discussions along with mathematical explanation on why the baseline shouldn't (or mustn't) be employed as a covariate, when analysing the change from baseline. From the other side, certain regulators, like those pharmaceutical drug approval bodies, often demand it. Today I am not asking whether it is valid or not, but rather if you saw any formal document, that would address that? At least with a single sentence.
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I found one: POINTS TO CONSIDER ON ADJUSTMENT FOR BASELINE COVARIATES
issued by The European Agency for the Evaluation of Medicinal Products Page 5
II.7. ‘Change from baseline’ analyses When the analysis is based on a continuous outcome there is commonly the choice of whether to use the raw outcome variable or the change from baseline as the primary endpoint. Whichever of these endpoints is chosen, the baseline value should be included as a covariate in the primary analysis. The use of change from baseline without adjusting for baseline does not generally constitute an appropriate covariate adjustment. Note that when the baseline is included as a covariate in the model, the estimated treatment effects are identical for both ‘change from baseline’ and the ‘raw outcome’ analysis. Consequently if the appropriate adjustment is done, then the choice of endpoint becomes solely an issue of interpretability.
Also: Adjusting for Covariates in Randomized Clinical Trials for Drugs and Biologics with Continuous Outcomes Guidance for Industry This is draft, but describes a common practice.
Many clinical trials use a change from baseline as the primary outcome measure. Even when 74 the outcome is measured as a change from baseline, the baseline value can still be used advantageously as a covariate
Issued by the FDA
https://www.fda.gov/regulatory-information/search-fda-guidance-documents/adjusting-covariates-randomized-clinical-trials-drugs-and-biologics-continuous-outcomes-guidance And the summary here: http://onbiostatistics.blogspot.com/2019/05/fda-and-ema-guidance-on-adjusting-for.html
Also:
Source: Guideline on adjustment for baseline covariates in clinical trials
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The FDA guideline has changed, but the overall idea remains: https://www.fda.gov/media/148910/download
Covariate adjustment is acceptable even if baseline covariates are strongly associated with each other (e.g., body weight and body mass index). However, adjusting for less redundant variables generally provides greater efficiency gains.
and
When using this approach, adjusting for the baseline value rather than (or in addition to) defining the primary endpoint as a change from baseline is generally acceptable
