Some analytes in blood serum samples, including persistent chemicals such as dioxins, furans, polychlorianted biphenyles (PCBs), and organochlorine pesticides, etc. are available in two ways: whole weight basis (i.e., per whole weight of serum) and lipid weight basis (i.e., per gram of total lipid).
For lipid soluable chemicals, the lipid adjusted measurement is recommended because of the lipid soluable nature of these chemical compounds and their concentrations in adipose tissue.
Total lipids (TL) in mg/dl for each serum specimen were estimated by using the summation of lipid values of individuals: TLs = (2.27 * Total Cholesterol) + Triglycerides + 62.3l; and serum specific gravity was also taken into account in the adjustment.
You don’t need to perform this adjustment because the unadjusted and lipid adjusted values are both included in the laboratory data set.
The concentrations of environmental chemicals per whole weight of serum are also on the laboratory file and can be used for comparison with other published studies that have investigated these chemicals.
The current NHANES urine collection protocol provides ‘spot’ urine samples because these are collected at different times of the day (depending on the examination session) and only one specimen is collected from each survey participant. The laboratory measures of environmental chemicals in urine are provided on the data files as concentrations per volume of urine. Each data set for environmental chemicals measured in urine, also includes a variable for urinary creatinine concentration.
Urine dilution may vary markedly from person to person, time to time, and because of other conditions, including fluid consumption, physical workload, and health. Creatinine is produced as a result of muscle metabolic processes, and excreted from the body at a fairly constant rate (though extreme diets may affect urine creatinine levels). The effect of urinary dilution can be accounted for by determining the amount of the environmental chemical per amount of urinary creatinine in a given volume of urine.
The equation for creatinine adjustment is:
Analyte concentration per gram of creatinine =
Concentration of environmental chemical in urine (wt/vol)
Concentration of creatinine in urine (wt/vol)
Creatinine is related to lean body mass and renal function of individuals, and varies by age, gender, and race/ethnicity group.
It is recommended that one compare the creatinine-corrected environmental chemical concentrations among individuals of similar demographic groups rather than the whole population because urinary creatinine levels differ according to age, gender, and race/ethnicity. Alternatively, multiple regression analyses can be conducted using urinary creatinine as an independent variable (in addition to variables for age, gender, and race/ethnicity), so that the environmental chemical concentrations comparisons can be based on adjustment for urinary dilution and demographic differences.