In the Star Trek universe: There are many types of tricorders, but the one used by medical personnel can diagnose diseases and collect other vital information about a patient. And it can do all this noninvasively, from a distance from the patient.
In real life: Perhaps the closest thing to a tricorder is a microfluidic lab-on-a-chip, which can analyze bodily fluids for early detection of diseases. Many companies are in competition for this space.
The problem, of course, is that a blood or saliva sample is necessary, and you'll never see "Bones" on Star Trek telling Kirk to go spit in a cup. No, we need a medical device that can take readings from a distance. Enter the Standoff Patient Triage Tool (SPTT, pictured below), being developed by the U.S. Department of Homeland Security's Science & Technology Directorate in partnership with the Technical Support Working Group, Boeing and Washington University's School of Medicine in St. Louis. The SPTT can take vital signs quickly from five to 40 feet away. It's being developed for triage in case of any disaster situation.
Also, at the National Space Biomedical Research Institute, a device dubbed the Venus prototype uses light to measure blood and tissue chemistry, metabolic rate and other parameters. But while it is noninvasive, you still have to place the device on the skin for it to work. That's fine if your patient is cooperative, but if, say, Spock is going through his every-seven-year Pon Farr, well good luck in getting him to hold still for it.