The European Uremic Toxin Work Group has defined the term middle molecule to be between 500 Daltons - 60 kD. There is still a strong thought that as-yet-unidentified unfiltered middle molecules may be a major reason for the unusually high cardiovascular mortality rate of ESRD patients.
Here is a helpful list of molecular weights of some common substances:
BUN (0.06 kD). Obviously, not a middle molecule; small, cleared rapidly.
Creatinine (0.113kD). Same as BUN: cleared rapidly.
Vitamin B12 (1.355 kD). A middle molecule of lower MW which is cleared by most modern dialyzers.
B2-microglobulin (11.8kD). One of the few middle molecules whose accumulation has been shown to be associated with a real medical condition: b2-amyloidosis. Newer generation dialyzers provide good clearance of b2-microglobulin and have made the clinical entity of b2-amyloidosis much more rare.
light chain (25kD). Light chains are small enough to get dialyzed--providing some rationale for dialyzing patients with paraproteinemias (though the clinical evidence for this being effective is not very good).
albumin (66kD). Albumin is not a middle molecule--it is not appreciably dialyzed off, nor would it be desirable to do so.