The question in the title is hard to answer, without a randomized controlled trial of some kind where the guidelines are followed in one group, and not in another. The whole point of most guidelines, though, is to put the best available evidence into a clinical pathway to be followed, so an RCT would have to randomize half the patients to not follow best practice, which would be ethically questionable.
Another way of evaluating their impact is to perform other types of studies, less scientifically convincing, but often the best that can feasibly be done.
In 2007, while I was chair of the Fetus and Newborn Committee, we produced a guideline on screening and management of hyperbilirubinemia. As most readers will know, jaundice in newborns is very frequent, almost always short-lived and benign, and frequently treated with phototherapy. Severe hyperbilirubinemia leading to neurological damage is very rare, but potentially avoidable. In 2006 Micheal Sgro and colleagues published data from the Canadian Paediatric Surveillance System (babies born in 2002-2004) that 1 infant per 2480 births developed severe jaundice (peak bili more than 425 micromol/L or an exchange transfusion); they also showed that 12% if them had acute neurological findings. From a different project investigating cases born in 2007 and 2008 they estimated that 1 in 44,000 births the baby had developed chronic bilirubin encephalopathy.
They have now repeated the earlier CPSP study: Sgro M, et al. Severe Neonatal Hyperbilirubinemia Decreased after the 2007 Canadian Guidelines. The Journal of pediatrics. 2016. In the new report there were 91 cases of severe hyperbilirubinemia country-wide, for a calculated incidence of 1 in 8600. One third of the previous incidence. This good news may, I would like to think, be at least partly due to the guidelines. Our guidelines recommended universal measurement of bilirubin, within the first 72 hours of life; they were based on the best available evidence, showing that visual inspection is unreliable, that many babies with severe hyperbilirubinemia had been discharged without having a measurement of bilirubin, and that a single measurement could predict with some accuracy which babies would become severely jaundiced.
The actual highest bilirubin in the 2 studies was almost identical at about 480, which hopefully means that the proportion of babies who develop chronic neurological problems will also decline by two thirds.
This has been achieved at, probably, very low cost. I say “probably” because the impacts on the use of phototherapy and duration of hospitalization are not consistent among studies examining the effects of introducing similar guidelines. Some have shown a small decrease in resource utilization, others a small, or not so small, increase in phototherapy and hospital stay. The guidelines were designed to limit phototherapy to those that really “needed” it (recognizing that very few would ever develop long-term harm), but then to do it properly and intensively. One of the studies showing an increase in phototherapy after guideline introduction in the USA noted that only half of the babies who received phototherapy actually should have had it, if the guidelines were really being followed.
Did the guideline make a difference? I like to think so, and I like to think that the difference was mostly a positive one. At least we seem to have moved in the right direction.