Table of Contents

Component Description

Specific goals of the serum iron status component include providing data in a nationally representative sample to: 1) assess overall iron status and function; 2) assess iron deficiency anemia and anemias of chronic diseases; 3) monitor trends in measures of nutritional status; and 4) assess and provide estimates of liver function in support of the liver elastography component.

Eligible Sample

Examined participants aged 12 years and older were eligible.

Description of Laboratory Methodology

All methods were measured on the Roche Cobas 6000 (c501 module) analyzer.

Iron (Frozen)

The Roche method of iron measurement is a three-step process using FerroZine reagent: Fe3+ is liberated from transferrin by acid/detergent, Fe3+ is reduced to Fe2+ by ascorbate, and the reduced iron then reacts with the FerroZine reagent to form a colored complex. The intensity of this final product is directly proportional to the iron concentration in the sample.

Unsaturated Iron Binding Capacity (UIBC)

In the first step of the measurement process in the Roche method for UIBC, excess Fe2+ is added to the specimen. In an alkaline environment it is converted to Fe3+ as it binds to the unbound endogenous transferrin. The unbound Fe2+ in the reagent then reacts with FerroZine reagent to form a colored compound. This is an inverse outcome: greater color development indicates the transferrin is more highly occupied by iron (low UIBC). Lesser color development indicates that the transferrin has more binding capacity available (high UIBC). Because of this characteristic and the calibration model, all results are multiplied by a factor (-1) to yield a positive result. This is a two-point, endpoint reaction, with measurement occurring at 546 nm (secondary wavelength 700 nm).

Total Iron Binding Capacity (TIBC)

Total Iron Binding Capacity (TIBC) was calculated indirectly using Iron (frozen), serum and Unsaturated Iron Binding Capacity, serum (UIBC):

Calculated TIBC (LBDTIB) = [Iron (frozen), serum (LBXIRN) + UIBC, serum (LBXUIB)]

Transferrin Saturation (%Sat)

Transferrin saturation (%Sat) value was calculated using Iron (frozen), serum and calculated Total Iron Binding Capacity (TIBC):

Calculated %Sat (LBDPCT) = [Iron (frozen), serum (LBXIRN) / TIBC (LBDTIB)] x 100

Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.

This is a new component in the NHANES 2017-2018 survey cycle.

Laboratory Method Files

A. Iron (Frozen) Laboratory Procedure Manual (February 2020)

B. UIBC Laboratory Procedure Manual (February 2020)

Laboratory Quality Assurance and Monitoring

Serum specimens were processed, stored, and shipped to the University of Minnesota, Minneapolis, MN for analysis.

Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM). Vials are stored under appropriate frozen (–30°C) conditions until they are shipped to University of Minnesota for testing.

The NHANES quality assurance and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Act mandates. Detailed QA/QC instructions are discussed in the NHANES LPM.

Mobile Examination Centers (MECs)

Laboratory team performance is monitored using several techniques. NCHS and contract consultants use a structured competency assessment evaluation during visits to evaluate both the quality of the laboratory work and the quality control procedures. Each laboratory staff member is observed for equipment operation, specimen collection and preparation; testing procedures and constructive feedback are given to each staff member. Formal retraining sessions are conducted annually to ensure that required skill levels were maintained.

Analytical Laboratories

NHANES uses several methods to monitor the quality of the analyses performed by the contract laboratories. In the MEC, these methods include performing blind split specimens collected on “dry run” sessions. In addition, contract laboratories randomly perform repeat testing on 2% of all specimens.

NCHS developed and distributed a quality control protocol for all CDC and contract laboratories, which outlined the use of Westgard rules (Westgard et al., 1981) when running NHANES specimens. Progress reports containing any problems encountered during shipping or receipt of specimens, summary statistics for each control pool, QC graphs, instrument calibration, reagents, and any special considerations are submitted to NCHS quarterly. The reports are reviewed for trends or shifts in the data. The laboratories are required to explain any identified areas of concern.

Data Processing and Editing

The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.

There were three additional variables created in this data file to convert the analyzed values into International System of Units (SI). These variables were created using the following formulas:

LBDIRNSI

The iron frozen, serum (LBXIRN) in ug/dL was converted to umol/L (LBDIRNSI) by multiplying by 0.1791.

LBDUIBSI

The Unsaturated Iron Binding Capacity, serum (LBXUIB) in ug/dL was converted to umol/L (LBDUIBSI) by multiplying by 0.1791.

LBDTIBSI

The calculated Total Iron Binding Capacity (LBXTIB) in ug/dL was converted to umol/L (LBDTIBSI) by multiplying by 0.1791.

Analytic Notes

Refer to the 2017-2018 Laboratory Data Overview for general information on NHANES laboratory data.

There are over 800 laboratory tests performed on NHANES participants. However, not all participants provided biospecimens or enough volume for all the tests to be performed. The specimen availability can also vary by age or other population characteristics. For example, in 2017-2018, approximately 80% of children aged 1-17 years who were examined in the MEC provided a blood specimen through phlebotomy, while 95% of examined adults age 18 and older provided a blood specimen. Analysts should evaluate the extent of missing data in the dataset related to the outcome of interest as well as any predictor variables used in the analyses to determine whether additional re-weighting for item non-response is necessary.

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.

Demographic and Other Related Variables

The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 2017-2018 Demographics File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample design variables. The recommended procedure for variance estimation requires use of stratum and PSU variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.

The Fasting Questionnaire File includes auxiliary information such as fasting status, length of fast, and the time of venipuncture.

The laboratory data file can be linked to other NHANES data file using the unique survey participant identifier (i.e., SEQN).

Detection Limits

The detection limits were constant for this analyte in the data set.

The lower limit of detection (LLOD, in ug/dL) for serum iron and UIBC:

Variable Name

Analyte Description

LLOD

LBXIRN

Iron frozen, Serum (ug/dL)

5

LBXUIB

Unsaturated Iron Binding Capacity (UIBC), Serum (ug/dL)

17

References

Codebook and Frequencies

SEQN - Respondent sequence number

Variable Name:
SEQN
SAS Label:
Respondent sequence number
English Text:
Respondent sequence number.
Target:
Both males and females 12 YEARS - 150 YEARS

LBXIRN - Iron frozen, Serum (ug/dL)

Variable Name:
LBXIRN
SAS Label:
Iron frozen, Serum (ug/dL)
English Text:
Iron frozen, Serum (ug/dL)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
11 to 481 Range of Values 5922 5922
. Missing 479 6401

LBDIRNSI - Iron frozen, Serum (umol/L)

Variable Name:
LBDIRNSI
SAS Label:
Iron frozen, Serum (umol/L)
English Text:
Iron frozen, Serum (umol/L)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
2 to 86.1 Range of Values 5922 5922
. Missing 479 6401

LBXUIB - UIBC, Serum (ug/dL)

Variable Name:
LBXUIB
SAS Label:
UIBC, Serum (ug/dL)
English Text:
Unsaturated Iron Binding Capacity (UIBC), Serum (ug/dL)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
12 to 602 Range of Values 5899 5899
. Missing 502 6401

LBDUIBLC - UIBC, Serum Comment Code

Variable Name:
LBDUIBLC
SAS Label:
UIBC, Serum Comment Code
English Text:
Unsaturated Iron Binding Capacity (UIBC), Serum Comment Code
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0 At or above detection limit 5893 5893
1 Below lower detection limit 6 5899
. Missing 502 6401

LBDUIBSI - UIBC, Serum (umol/L)

Variable Name:
LBDUIBSI
SAS Label:
UIBC, Serum (umol/L)
English Text:
Unsaturated Iron Binding Capacity (UIBC), Serum (umol/L)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
0.18 to 107.82 Range of Values 5899 5899
. Missing 502 6401

LBDTIB - Total Iron Binding Capacity TIBC (ug/dL)

Variable Name:
LBDTIB
SAS Label:
Total Iron Binding Capacity TIBC (ug/dL)
English Text:
Total Iron Binding Capacity TIBC (ug/dL)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
139 to 621 Range of Values 5893 5893
. Missing 508 6401

LBDTIBSI - Tot Iron Binding Capacity TIBC (umol/L)

Variable Name:
LBDTIBSI
SAS Label:
Tot Iron Binding Capacity TIBC (umol/L)
English Text:
Total Iron Binding Capacity (TIBC) (umol/L)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
24.89 to 111.22 Range of Values 5893 5893
. Missing 508 6401

LBDPCT - Transferrin Saturation (%)

Variable Name:
LBDPCT
SAS Label:
Transferrin Saturation (%)
English Text:
Transferrin Saturation (%)
Target:
Both males and females 12 YEARS - 150 YEARS
Code or Value Value Description Count Cumulative Skip to Item
2 to 92 Range of Values 5893 5893
. Missing 508 6401