Microalbuminuria, 24-Hour Collection, Urine

CPT CODE:

USEFUL FOR:

Evaluating diabetic patients to assess the potential for early onsetof nephropathy

SPECIMEN REQUIRED:

5 mL from a 24-hour urine collection. No preservative. See "Urine Preservatives" in Special Instructions for multiple collections. Mix well before taking 5-mL aliquot. Send specimen refrigerated in a plastic, 13-mL urine tube. Note:    24-Hour volume is required on request form for processing.            If specimen is not a 24-hour timed collection, specimen will be              processed but reference values will not apply.   Urine Preservative Collection OptionsNote:    The addition of preservative or application of temperature              controls must occur within 4 hours of completion of the              collection.
Ambient:                PreferredRefrigerate:           YesFrozen:                 Yes6N HCl:                 No50% Acetic Acid:   NoNa(2)CO(3):          NoToluene:                Yes6N HNO(3):           NoBoric Acid:            YesThymol:                 Yes

TRANSPORT TEMPERATURE:

Refrig\Ambient OK\Frozen OK

CLINICAL INFORMATION:

Albumin excretion increases in patients with diabetes who are destinedto develop diabetic nephropathy. More importantly, at this phase ofincreased albumin excretion before overt proteinuria develops, therapeutic maneuvers can be expected to significantly delay, or possibly prevent, development of nephropathy. These maneuvers include aggressive blood pressure maintenance (particularly with angiotensin-converting enzyme [ACE], inhibitors), aggressive blood sugar control, and possibly decreased protein intake. Thus, there is a need for addressing small amounts of urinary albumin excretion (in the range of 30-300 mg/day, ie, microalbuminuria)
The National Kidney Foundation convened an expert panel to recommend guidelines for the management of patients with diabetes and microalbuminuria. These guidelines recommend that all type 1 diabetic patients older than 12 years and all type 2 diabetic patients younger than 70 years should have their urine tested for microalbuminuria yearly when they are under stable glucose control.(1)
The preferred specimen is a 24-hour collection, but a 10-hourovernight collection (9 p.m. to 7 a.m.) or a random collection areacceptable. Recent studies have shown that correcting albumin for creatinine excretion rates has similar discriminatory value with respect to diabetic renal involvement, and it is now suggested that an albumin/creatinine ratio from a random urine specimen is a valid screening tool.(2) Several studies have addressed the question of whether this needs to be a fasting urine, an exercised urine, or an overnight urine specimen. From these studies, it is clear that the first-morning urine specimen is lesssensitive, but more specific. A positive result should be confirmed by afirst-morning random or 24-hour timed urine specimen.
Studies have also shown that microalbuminuria is a marker ofgeneralized vascular disease and is associated with stroke and heartdisease. 

CLINICAL INTERPRETATION:

An albumin excretion rate >30 mg/24 hours is considered to bemicroalbuminuric. By definition, the upper end of microalbuminuriais thought to be 300 mg/24 hours. Although this level has not beenrigorously defined, it is felt that at this level it is more difficult tochange the course of diabetic nephropathy. We have establishednormal values in our laboratory and agree with the 30 mg/24 hourlevel. A normal excretion rate of 20 mcg/min has also beenestablished in the literature and is consistent with our data. Thus,microalbuminuria has been defined at 30 mg/24 hours to 300 mg/24hours.
The literature has defined the albumin/creatinine ratio (mg/g) <17as normal for males and <25 for females(2) and is consistent withour normal data. A ratio of albumin to creatinine of > or =300indicates overt albuminuria. Thus, microalbuminuria has beendefined as an albumin/creatinine ratio of 17 to 299 for malesand 25 to 299 for females.
Due to biologic variability, any patient who has an albumin/creatinineratio or urinary albumin excretion rate in the positive microalbuminuriarange should have this confirmed with a second specimen. If there isdiscrepancy, a third specimen is recommended. If 2 of 3 results arein the positive microalbuminuria range, this is evidence for incipientnephropathy and warrants increased efforts at glucose control,aggressive blood pressure control, and institution of therapy withan ACE inhibitor (if the patient can tolerate it).
On 02/10/2009, the reagent system used for the immunoturbidimetricalbumin assay was changed. This resulted in a small but statisticallysignificant change in albumin measurements. To directly compareresults after the date of this method change to previous values the oldurinary albumin concentration should be multiplied by 1.11. Given thenormal physiologic variability of albumin excretion in individuals andthe small magnitude of this correction factor, in most instances thiscalculation will not be necessary for clinical management of patients.

REFERENCE VALUES:

24-Hour excretion:  <30 mg/24 hours

Excretion rate:  <20 mcg/min