At my facility, critical values are in red on the computer. Lab will also call us with critical results. Now some values are to be expected (such as critically high creatinine/BUN levels in patients with CKD/ESRD, on dialysis, etc) but if it's critical, we still have to report it. Policy at my facility states we must notify the MD within 1 hour of receiving the critical value.
In our ICU if we suspected a way out of whack lab when we sent the blood, we'd watch the computer like a hawk for the results. When the lab finally called us, we enjoyed saying: "Okay. I treated that 45 minutes ago, but thanks for calling!" Sometimes, the value would be 'critical' for days, but improving, then would could say: " Hey! It's getting better! Thanks for calling. "
Of course, every once in a while, we'd get the call and say "WHAT? OH &$*#!" (but not often)
I also work in LTC but with ventilator dependent patients. We get critical labs all the time and they are always called in to us directly, and the lab has to verify and document who they reported the critical value to. Our lab aso labels values with CH for critically high and CL for critically low.
At our facility it is almost impossible to overlook a crtical lab and assume it is similar elsewhere. What can get tricky is determining what labs that are out of rang but not critical warrant notifying the MD. This comes with experience and if you are in doubt I would always call to cover yourself. I have been reemed out by a few MDs about labs when I first started but just don't let it bother you if it happens. They are getting paid to care for the pts, even if they're on call, is how I look at it. Better safe than sorry.