National Health and Nutrition Examination Survey

Urinary albumin is measured. Related survey questionnaire data include information on analgesic product use and incontinence. Urinary creatinine is measured. Urine creatinine concentrations from these specimens have been used (instead of a 24-hour volume correction) to correct for the degree of dilution or concentration of some of the urine analytes, such as the phenols or microalbumin.

Participants aged 6 years and older.

Urine specimens are processed, stored and shipped to University of Minnesota, Minneapolis, MN. Detailed specimen collection and processing instructions are discussed in the NHANES Laboratory/Medical Technologists Procedures Manual (LPM).

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.

Increased microalbuminuria is a sign of renal disease and may be predictive of nephropathy risk in patients with insulin-dependent diabetes. 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 Beckman Synchron CX3 Clinical Analyzer

Creatinine analysis uses a Jaffé rate reaction, in which creatinine reacts with picrate in an alkaline solution to form a red creatinine-picrate complex. The reaction is measured with a CX3 analyzer. The rate of the color development is measured 25.6 sec after sample injection at 520 and 560 nm. The rate difference between the two wavelengths is proportional to the concentration of creatinine in the reaction cup. The procedures described below are the standard protocols of the Fairview University Medical Center (Creatinine Measurement Module Operating and Service Instructions, Beckman ASTRA. Brea (CA): Beckman Instruments, Inc., 1979; Operating and Service Instructions, Beckman ASTRA. Brea (CA): Beckman Instruments, Inc., 1986; Maintenance Guide, Beckman ASTRA. Brea (CA): Beckman Instruments, Inc., 1982; Tietz NW, editor, Textbook of Clinical Chemistry. Philadelphia: WB Saunders Company, 1986; 775–1392.; Kaplan LA, Pesce AJ, editors, Clinical Chemistry Theory, Analysis and Correlation. St. Louis: CV Mosby Company, 1984:416–1261).

Creatinine, the waste product derived from creatine, is released into the plasma at a relatively constant rate. The amount of creatinine per unit of muscle mass is constant; therefore, creatinine is the best indicator of impaired kidney function.

Urinary Creatinine using 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.

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 may be measured in both serum and urine. Creatinine measurement is useful in the diagnosis and treatment of renal diseases, in monitoring renal dialysis, and as a calculation basis for other urinary analytes (e.g. total protein, microalbumin). The ratio of urine albumin to urine creatinine is used to predict nephropathy risk in diabetic patients. (For the NHANES IV survey, urine creatinine is used as a reference analyte against which are measured other urine analytes, such as pesticides and heavy metals, and urine albumin/creatinine ratio).

There were no changes (from the previous 2 years of NHANES) to the lab site. There were changes (from the previous 2 years of NHANES) to equipment and lab method for urinary creatinine. During 2007-2008 urinary creatinine was measured using Roche/Hitachi Modular P Chemistry Analyzer and using Beckman Synchron CX3 Clinical Analyzer
A detailed description of the laboratory method used can be found on the NHANES website.

Read the General Documentation of Laboratory Data file for detailed data processing and editing protocols. The analytical methods are described in the Analytic methodology section.

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

A detailed description of the quality assurance and quality control procedures can be found on the NHANES website.

Urine Creatinine Adjustment to compare 2007-2008 to 2005-2006

There was a change in the instrumentation and method for urine creatinine from 2006 to 2007. Prior to 2007, the urine creatinine was performed on the Beckman CX3 using a Jaffe reaction. From 2007 forward, the urine creatinine was performed on the Roche ModP using an enzymatic (creatinase) method. The preferred method is the enzymatic method since the Jaffe method is subject to more interference.

Crossover studies were performed between the ModP and CX3 methods. Initial crossover analysis revealed that the mean CX3 was 134.7 mg/dL and the mean ModP was 129.7 mg/dL (an average bias of -3.7%).

A second crossover was designed more optimally in the selection of the specimens. The specimens were from all age groups, equal number of males and females, at least 25% Non-Hispanic Blacks, specimens were collected over the entire period of 2005-2006 with a more uniform distribution of lab values, and the crossover was analyzed for a 1 month period. The mean of data for the crossover CX3 was 140.6 mg/dL and the mean ModP was 134.9 mg/dL (an average bias of -4.1%).

The distribution of urine creatinine was not Gaussian and the square root transform of the data produced a normal distribution. This improved the comparison of the adjusted 2005-2006 urine creatinine distribution to the 2007 data. A piecewise square root transformed adjustment of 2005-2006 data further improved the comparison of 2005-2006 to 2007. The following equations are recommended to adjust urine creatinine prior to 2007 to compare with urine creatinine from 2007 forward. The units of urine creatinine are mg/dL:

Urine Creatinine < 75: Y(adjusted Creatinine) = [1.02*sqrt(X, unadjusted Creatinine) – 0.36]**2

Urine Creatinine 75 to < 250: Y(adjusted Creatinine) = [1.05*sqrt(X, unadjusted Creatinine) – 0.74]**2

Urine Creatinine >=250: Y(adjusted Creatinine) = [1.01*sqrt(X, unadjusted Creatinine) – 0.10]**2

A weighted analysis of NHANES 2007-2008 vs. 2005-2006 on urine creatinine was performed applying the piecewise regressions above. This revealed a lower bias where the mean of adjusted data for 2005-2006 creatinines was 123.31 mg/dL and the mean of the 2007-2008 creatinines was 122.36 mg/dL (an average bias of -0.77%).

The analysis of NHANES laboratory data must be conducted with the key survey design and basic demographic variables. The NHANES Household Questionnaire Data Files contain demographic data, health indicators, and other related information collected during household interviews. They also contain all survey design variables and sample weights for these age groups. The phlebotomy file includes auxiliary information such as the conditions precluding venipuncture. The household questionnaire and phlebotomy 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 Analytic Guidelines for further details on the use of sample weights and other analytic issues. The Analytic Guidelines are available on the NHANES website.