This article was first published on November 15, 2022. It was last reviewed and updated on November 15, 2022.
On other pages on the website, we have discussed the importance of serum creatinine—and estimated glomerular filtration rate (eGFR) that is calculated from the serum creatinine—in evaluating kidney function. For links to those articles, please see Related Pages, below.
Normally, the kidney doesn’t allow more than a small trace of albumin to go out in the urine. But, when kidney damage is present, more albumin may be found in the urine (“albuminuria”). This is because of damage to the glomeruli, allowing albumin to “leak” into the urine in larger quantities than normal.
Two tests that may come to mind are either not enough or not convenient:
1. A routine urinalysis checks for the presence or absence of protein. But even if the standard, routine urinalysis does not show protein, microalbuminuria may still be present.
2. Yes, a 24-hour urine albumin test can measure how much albumin is being excreted in the urine per day. But collecting a 24-hour urine sample is VERY inconvenient.
The problem is that if a single, random sample of urine is used, there is no normal reference range for urine albumin. Fortunately, we have a solution to this problem. We can test a single, random urine sample for the RATIO of albumin to creatinine. We can see this test on the website of Labcorp (a large chain of laboratories in the US) at this link.
To summarize, important tests of kidney function that we can and should order include:
1. Serum creatinine/ eGFR (included in a “Basic metabolic panel”)
2. Urinalysis–complete with microscopic examination (example)
3. Albumin-creatinine ratio (random urine)
Important! The urine albumin-creatinine ratio can be elevated even when the eGFR is normal.
Interpretation
The term microalbuminuria is no longer used (KDIGO CKD Work Group, 2013). Instead, the albumin-creatinine ratio is divided into the following 3 categories (KDIGO CKD Work Group, 2013):
Normal to mildly increased: 0 to 29 mg/g
Moderately increased: 30 to 300 mg/g
Severely increased: more than 300 mg/g
In a simple way, we can remember that a urine albumin-creatinine ratio of 30 or more is a cause for concern.
Why the urine albumin-creatinine ratio is important
Convenience
The excretion of creatinine in the urine is relatively constant throughout the day. So, if the urine is concentrated, the urine creatinine concentration will go up, and vice versa. This is why using the ratio of albumin to creatinine in the same urine sample allows us to take into account how concentrated the urine is. Due to being able to check the urine albumin-creatinine ratio, we don’t have to collect a 24-hour urine sample (which is very inconvenient) to quantify albuminuria.
Prognostic significance
An increased urine albumin-creatinine ratio is associated with an increased risk of end-stage kidney disease, and cardiovascular mortality even if the eGFR is normal.
Clinical decision-making
In a patient with a mild decrease in the eGFR (which is not at all rare in my own patients), if the urine albumin-creatinine ratio is increased, this indicates that the renal disease is significant and the patient should be referred to a kidney specialist for evaluation and treatment.
Related Pages
Kidney (renal) function tests
What is the estimated GFR (eGFR)?
What serum creatinine is and why it is important
Factors that can affect serum creatinine
Can creatine supplements lead to “false” elevation in serum creatinine?
Why it is important to longitudinally chart serum creatinine and eGFR values
Another way in which charting serum creatinine values can be helpful
How to know if the BUN and creatinine are elevated due to dehydration
In suspected kidney disease, check the urine albumin-creatinine ratio
24-hour urine for creatinine clearance
How to collect a 24-hour urine sample
Kidney (renal) disease and psychiatric medications
What are the stages of chronic kidney disease?
Lithium-induced renal impairment: Clinical recommendations
Bupropion in persons with renal disease
Antidepressants in renal (kidney) disease
Psychotropic medications that are not metabolized (mostly excreted unchanged)
Mirtazapine and renal transplant
Lamotrigine in persons with renal disease (coming soon)
References
Gaitonde DY, Cook DL, Rivera IM. Chronic Kidney Disease: Detection and Evaluation. Am Fam Physician. 2017 Dec 15;96(12):776-783. PMID: 29431364.
Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl. 2013;3:1-150. Available at https://kdigo.org/wp-content/uploads/2017/02/KDIGO_2012_CKD_GL.pdf. Last accessed on November 15, 2022.
Levey AS, Coresh J. Chronic kidney disease. Lancet. 2012 Jan 14;379(9811):165-80. doi: 10.1016/S0140-6736(11)60178-5. Epub 2011 Aug 15. PMID: 21840587.
Levey AS, de Jong PE, Coresh J, El Nahas M, Astor BC, Matsushita K, Gansevoort RT, Kasiske BL, Eckardt KU. The definition, classification, and prognosis of chronic kidney disease: a KDIGO Controversies Conference report. Kidney Int. 2011 Jul;80(1):17-28. doi: 10.1038/ki.2010.483. Epub 2010 Dec 8. Erratum in: Kidney Int. 2011 Nov;80(9):1000. Erratum in: Kidney Int. 2011 Nov 1;80(9):1000. PMID: 21150873.
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Is this something we should be checking on all our patients on lithium as you mentioned that increased urine albumin-creatinine ratio is associated with an increased risk of end-stage kidney disease, and cardiovascular mortality even if the eGFR is normal. ? Would it potentially pick up concerns before there is a significant issue (and even if GFR is normal)