But is there any clinical advantage to using one or the other? I'm not a biochemist, I just run my wife's family practice clinic (or surgery to use an archaic British term

). I don't see any evidence, however, that the British units of measurement are somehow more accurate or, better yet, make diagnosis any better. It's just a different way of doing it and both work equally well. I can understand how it would be a hassle for British doctors to convert units and such, but we Americans aren't in the habit of changing our ways to suit the conventions of other countries. My god, we still use the archaic inches/feet system! So if you are saying we are archaic because we use old units, well you have a very good point! But "old measurement units" does not necessarily translate to "outdated Laboratory Science."
When the US uses different units than the rest of the world, and we have doctors that work in both countries and patients that are also in both countries, differences in units can lead to accidents.
Most medics pay no attention to the units, it is the number they are used to.
This is especially critical when dealing with drugs and drug overdoses. This has been an issue with paracetamol/acetaminophen in particular (which is the most common drug people overdose on), that I think has resulted in the death of a few patients.
The US is also not consistent, so for example, they use mmol/L for things like sodium and potassium, but mg or ng/dl for everything else.
It isn't consistent with modern scientific practice.
I will see if I can find a set of results somewhere, I am fairly positive I have saved a few US case studies.
As I mentioned before, it is things like still relying on total hormone levels with a binding globulin to get an index.
Take testosterone for example, historically, it has been notoriously difficult to measure (this has improved within the last year in the UK).
Testosterone is mostly bound to sex hormone binding globulin (SHBG), so this would be measured to created a free testosterone level (which is not measured directly at all).
Two somewhat dodgy and error ridden analytes combined only compound the error. As well, it is totally unnecessary clinically, with the rare exception of odd genetic disorders of SHBG, for 99.9% of the population, if your total testosterone is high, your free T is high, if your total testosterone is low, your free T is also low.
This logical application of laboratory medicine to the clinical practice is typically not done in the US, the sentiment is that more information is better, even if the information is error ridden and, really, clinically unnecessary in 99.9% of cases.
This is just one hormone, but it is also hit and miss to even a worse extent for thyroid function tests where I had to dig out textbooks from the 1970s to even figure out what the hell a reverse T3 is.
I am also not entirely sure how well established the external QC system is for the US, and the phenomenal number of consumer direct on-line laboratories with bizarre and often useless 'panels' for screening for everything from cancer to autism.