Microalbuminuria refers to tiny quantities of albumin in the urine. Usually these quantities are below the limits of detection of conventional urine dipsticks. Finding this urinary abnormality means different things in different patients. In patients without any risk factors for cardiovascular disease, finding microalbuminuria might mean only a very mild renal abnormality has been detected without much clinical significance.
However, in patients with risk factors for vascular disease (hypertension, diabetes, hyperlipidemia), there is a large body of evidence to suggest that microalbuminuria is associated with increased cardiovascular risk. Therefore, one might treat patients with this finding and with risk for vascular disease as high risk when considering therapy and targets of these therapies.
Despite the known associations, mass screening is probably only indicated in patients with diabetes and not for general vascular risk stratification.
24 hour urines are useful for quantifying large amounts of protein (>1-2 g/day), though you can estimate this with a albumin:creatinine ratio as well. The one issue to remember is that MAU only measures albumin. If you want to test for non-albumin protein (like in myeloma), then either perform a urine protein electrophoresis or if there is large disparity between and albumin and protein to creatinine ratio, this suggests a significant non-albumin protein is present.