As describe by Cook and Campbell (1979) there are four components of validity: internal validity, external validity, statistical conclusion validity and construct validity. Internal validity relates to whether there is a causal relationship between an exposure (e.g. contact precautions) and an outcome (MRSA infections) that is free of bias. Randomization is thought to improve internal validity through reduction in confounding associated with unmeasured factors. External validity describes the generalizability of the findings to other populations (quasi-experimental studies typically have higher generalizability vs RCTs). Statistical validity is concerned with covariation between exposure and outcome and strength of the association (e.g. Type 1 and Type II error). All types of studies can have high or low statistical validity – it is independent of study design.
Finally, construct validity describes whether a test measures what it claims to be measuring. For example, if you claim a person is ESBL negative, is she actually free of ESBL colonization or infection and will not develop an ESBL infection in the future. As you can see, without high construct validity, all the other components of validity are unimportant. If your study design or microbiological method is unable to detect ESBL properly, it is irrelevant if you’ve completed a cluster-RCT or whether your p-value is significant. Thus, validity theory defines construct validity as the primary concern, subsuming all other types of validity evidence.
Which brings me to studies claiming contact precautions don’t prevent MRSA, ESBL or VRE. Let’s think about MRSA. If an MRSA negative patient is admitted to a hospital with a 4 day length of stay. On average (normal distribution) that patient would be expected to acquire MRSA at the end of day 2. Thus, they would have to go from acquisition to infection over the next two days prior to discharge for most studies to prove she didn’t acquire MRSA. Would two days even be long enough for her to have a positive nasal swab? So, how sensitive are surveillance cultures or clinical cultures at detecting this event. I’d suggest not sensitive at all. Thus, to have strong construct validity in any study looking at the benefits of eliminating contact precautions, the study would have to track patients for a prolonged period of time (months) and in particular look at infections that manifest during subsequent admissions including those to long term care facilities.
So, before we can make claims about the benefits or lack of benefits of infection control interventions we need to design studies with high construct validity. I would suggest that our ability to respond to current MDR-bacterial pandemics will foremost depend on us designing studies with strong construct validity. Pathogens will continue to harm our patients until we identify methods to halt their spread. The current trend towards infection control nihilism that is manifesting with those eliminating contact precautions based on studies with poor construct validity and typically very poor statistical validity (underpowered) is harming our patients – often after they are discharged from our facilities.