National Health and Nutrition Examination Survey

Urinary albumin and urinary creatinine are measured in a random urine collected in the MEC (first collection) and a first morning void urine collected by the participant at home (second collection). Urine albumin-creatinine ratio is used to classify stages of chronic kidney disease. In addition, the urine creatinine is used to standardize the collection of urine analytes such as environmental chemicals.

Albumin-Creatinine Ratio (ACR):
Chronic kidney disease (CKD) is a serious condition associated with premature mortality, decreased quality of life, and increased health-care expenditures. Untreated CKD can result in end-stage renal disease and necessitate dialysis or kidney transplantation. Risk factors for CKD include cardiovascular disease, diabetes, hypertension, and obesity.

Persistent albuminuria is used to determine kidney damage for categorizing persons as having stage 1 and stage 2 CKD. Two urine samples are needed to assess persistent albuminuria and confirm the presence of kidney damage.

Increased microalbuminuria is a sign of renal disease and may be predictive of nephropathy risk in patients with insulin-dependent diabetes. Various large cohort studies have shown that microalbuminuria is a strong risk predictor for cardiovascular morbidity and all-cause mortality. Because urinary albumin excretion follows a circadian rhythym, the preferred method to collect urine for albumin assessment is to collect a 24 hour urine specimen. However, a 24-hour urine collection is inconvenient to obtain in NHANES for logistical reasons. Therefore, measurement of urinary albumin and creatinine concentrations are performed and an albumin:creatinine ratio (ACR) was determined from both a random urine and a first morning void.

Creatinine is produced by creatine and creatine phosphate as a result of muscle metabolic processes. Creatinine is the waste product derived from muscle creatinine and is released into the blood at a relatively constant rate. It is then excreted by glomerular filtration during normal renal function. The amount of creatinine per unit of muscle mass is constant; therefore, increased blood creatinine is the best indicator of impaired kidney function

Creatinine measurement is useful in the diagnosis and treatment of renal diseases, in monitoring renal dialysis, and is used to standardize other urinary analytes (e.g. environmental chemicals). The ratio of urine albumin to urine creatinine is used to predict nephropathy risk in diabetic patients.

Since the ACR depends not only on urinary albumin but also on urinary creatinine excretion, it will be affected by gender and age because muscle mass is lower in females than in males and decreases with age. The urine albumin-creatinine ratio is calculated as:

ACR(mg/g) = urine albumin (mg/dL) / Urine creatinine (g/dL)

Participants aged 6 years and older.

Urine specimens are processed, stored and shipped to University of Minnesota, Minneapolis, MN for analysis.

Urinary albumin
A solid-phase fluorescent immunoassay for the measurement of human urinary albumin is described by Chavers et al. (Chavers, BM, Kidney Int. 1984; 25:576–578). The fluorescent immunoassay is a non-competitive, double-antibody method for the determination of human albumin in urine. Antibody to human albumin is covalently attached to derivatized polyacrylamide beads. The solid-phase antibody is reacted with a urine specimen, and the urine albumin-antigen complexes with the solid-phase antibody. This complex then reacts with fluorescein-labeled antibody. The unattached fluorescent antibody is then removed by washing during centrifugation. The fluorescence of the stable solid-phase antibody complex is determined with a fluorometer; the fluorescence is directly proportional to the amount of urine albumin present. The standard curve is 0.5–20 μg/mL of albumin.

Results of the fluorescent immunoassay (FIA) are reproducible, and the test is accurate and sensitive for the detection of human urinary albumin excretion. It is especially useful for the measurement of low levels of urinary albumin not detectable by dipstick methods. The FIA assay resembles the radio-immunoassay (RIA) in technique and sensitivity without the potential health hazards associated with the handling of isotopes in the laboratory (Chavers, BM, Kidney Int. 1984; 25:576–578). 

Urinary Creatinine using the Roche/Hitachi Modular P Chemistry Analyzer
In this enzymatic method creatinine is converted to creatine under the activity of creatininase. Creatine is then acted upon by creatinase to form sarcosine and urea. Sarcosine oxidase converts sarcosine to glycine and hydrogen peroxide, and the hydrogen peroxide reacts with a chromophore in the presence of peroxidase to produce a colored product that is measured at 546 nm (secondary wavelength = 700 nm). This is an endpoint reaction that agrees well with recognized HPLC methods, and it has the advantage over Jaffe picric acid-based methods that are susceptible to interferences from non-creatinine chromogens.

There were no changes (from the previous 2 years of NHANES) in the laboratories performing the analyses.

Detailed instructions on specimen collection and processing can be found in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM).

Read the General Documentation on Laboratory Data file for detailed data processing and editing protocols. The analytical methods are described in the Description of Laboratory Methodology section above.

Two calculated variables, URDACT and URDACT2, were created in this data file:

The random (first collection) urine albumin (URXUMA) in ug/mL and urine creatinine (URXUCR) in mg/dL were converted to the albumin/creatinine ratio (URDACT) in mg/g.:

 URDACT = URXUMA/URXUCR

The morning void urine (second collection) albumin (URXUMA2) in ug/mL and second urine creatinine (URXUCR2) in mg/dL were converted to the second albumin/creatinine ratio (URDACT2) in mg/g:

URDACT2 = URXUMA2/URXUCR2

The NHANES quality control and quality assurance protocols (QA/QC) meet the 1988 Clinical Laboratory Improvement Act mandates. Detailed QA/QC instructions are discussed in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM). Read the General Documentation on Laboratory Data file for detailed QA/QC protocols.

Analytic note on comparing the urine albumin-creatinine ratio of the random urine (first collection) and the follow-up first-morning void urine (second collection)

Persistent albuminuria, as measured by the urine albumin-creatinine ratio (ACR) in two urines from an individual, is used to determine the prevalence of stages 1 and 2 of chronic kidney disease. In NHANES 2009-2010, two urine samples were collected from participants ages 6 years and older. A random urine was initially collected in the NHANES mobile examination center (MEC). Participants were then asked to collect a first-morning void urine in their home within 10 days of the MEC examination.

The random urine (first collection) has a higher mean ACR compared with the mean of the first-morning void urine (second collection) because of factors such as orthostatic (postural) proteinuria and exercise. For participants 20 years and older in NHANES 2009-2010 (n=5247), the weighted (using MEC examined sample weights) mean random ACR (first collection) was 23.7 mg/g and the mean first-morning void ACR (second collection) was 15.9 mg/g. The median random urine ACR (first collection) was 6.0 mg/g and the first-morning void ACR (second collection) was 4.5 mg/g. The percent of participants with an ACR 30 mg/g or greater was 7.7% for the random urine (first collection) and 3.4% for the first-morning void (second collection).

It is recommended that data users carefully interpret the differences between the random and the follow-up first-morning void urine results to estimate the prevalence of persistent albuminuria. The users should take into consideration the expected and observed differences between the random and first-morning void results because of the difference in collection times. In NHANES III, a study of two random urines was used to estimate persistent albuminuria. The comparison of estimates of the prevalence of albuminuria from the random and follow-up first-morning void urines may be of interest since both types of specimens are usually collected in the clinical evaluation of albuminuria.

NHANES Survey Design:

The analysis of NHANES laboratory data must be conducted with the key survey design and basic demographic variables. The Demographic file contains: Status Variables providing core information on the survey participant including examination status, Recoded Demographic Variables including age, gender, race etc., and Interview and Examination Sample Weight Variables and Survey Design Variables. The Questionnaire Data Files contain socio-economic data, health indicators, and other related information collected during household interviews. The Phlebotomy Examination file includes auxiliary information on duration of fasting, the time of day of the venipuncture, and the conditions precluding venipuncture. The Demographic, Questionnaire and Phlebotomy Examination files may be linked to the laboratory data file using the unique survey participant identifier SEQN.

Exam sample weights should be used for analyses. Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial  for further details on the use of sample weights and other analytic issues.