Variables confounding

Anticholinergic agents should not be abruptly withdrawn but use may be a confounding variable in the setting of hyperthermia... 

Another category of potential confounding variables includes dosing issues. Caffeine researchers have administered a wide variety of experimental dosages, an issue related obviously to physiological effects but also of interest to those concerned with caffeine doping. The timing of the... 

Another occupational study compared 53 lead-exposed male workers (2 nonwhite, 51 white) (mean PbB, 47.4 pg/dL range, 44-51 pg/dL) from a plant processing lead and cadmium compounds with a control group of 52 workers (8 nonwhite, 44 white) (mean PbB, 8.1 pg/dL, with none exceeding 20 pg/dL) from a nonlead industry (de Kort et al. 1987). Blood pressure levels were positively correlated with PbB and urine cadmium levels, but not with blood cadmium levels. The correlation for systolic blood pressure and PbB level remained significant after controlling for confounding variables. 

In adults, a study of 75 autopsies of persons who had resided in a soft-water, leached soil region of North Carolina found a positive correlation between lead level in the aorta and death from heart-related disease (Voors et al. 1982). The association persisted after adjustment for the effect of age. A similar correlation was found between cadmium levels in the liver and death from heart-related disease. (Aortic lead and liver cadmium levels were considered to be suitable indices of exposure.) The effects of the two metals appeared to be additive. Potential confounding variables other than age were not included in the analysis. The investigators stated that fatty liver (indicative of alcohol consumption) and cigarette smoking did not account for the correlations between lead, cadmium and heart-disease death. 

These three studies indicate that exposure to low levels of lead may impact negatively upon children s hearing. However, the authors of the Robinson study did not state whether age and other possible confounding variables were controlled for. Similarly, in the NHANES study, age may have been a confounding variable. 

Cancer. The information available on the carcinogenicity of lead in occupationally exposed humans is limited in its usefulness because the lead compound(s), the route(s) of exposure, and the levels of exposure were not always reported. Furthermore, concurrent exposure to other chemical (including arsenic, particularly in lead smelters) and confounding variables, such as smoking, were often not evaluated. Therefore, the data currently available do not support an assessment of the potential carcinogenic risk of lead in humans. 

Drug therapy optimization in ARF is a challenge. Confounding variables include residual drug clearance, fluid accumulation, and use of RRTs. 

If this model is selected, one must then decide what variables to use for the ordinate and the abscissa. The parameters must be dose-sensitive, free of confounding variables, easily determined and preferably linear. We have evaluated this approach for estimating the bioavailability of calcium in mechanically deboned meat products (11). Typically, correlations between various bone parameters and dietary calcium are very high (r = 0.943 to 0.999). This is consistent with what others have found for similar parameters (46,47). These correlations are also similar to the those (r = 0.947 to 0.982) between the amount of calcium consumed and calcium retained (11) a good index procedure. 

Studies to date have demonstrated an extremely weak correlation (if any) between photochemical-oxidant pollution and hospital admissions. If such correlations do exist, observations must be carried out over considerably longer periods with adequate provisions for controlling confounding variables, such as meteorologic factors. 

Hepatic Effects. Data in humans are limited to one study which reported significant dose-related increases in the concentration of serum bile acids in workers after inhalation exposure to hexachlorobutadiene (0.005-0.02 ppm) (Driscoll et al. 1992). The practical importance of this finding is reduced because workers were also potentially exposed to other solvents (carbon tetrachloride and perchloroethylene) and background information on other confounding variables was minimal. No studies were located regarding other hepatic effects in humans. 

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