Evaluating Confounding

Confounding occurs when an observed association between the suspected risk factor and outcome is due to a different risk factor, which is the cause of the disease. 

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Preexisting antimicrobial resistance is an increasing cause of treatment failure and is estimated to account for up to 70% of all treatment failures. Geography is the most important factor in HP resistance. Metronidazole-resistant strains are more prevalent in Asia (85%) than North America (30%).15 Primary resistance to amoxicillin and tetracycline remains low in both the United States and Europe. Clarithromycin resistance rates are estimated to be approximately 10% in the United States. Another confounding factor when evaluating potential antibiotic resistance is that culture and sensitivity studies are not routinely performed with HP infection. 

The analysis carried out by Schwartz (1994) included a total of eight studies, three longitudinal and five cross-sectional, relating blood lead to Full Scale IQ in school age children. To evaluate potential confounding, the baseline meta-analysis was followed by sensitivity analyses in order to contrast results... 

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. 

Epidemiological and Human Dosimetry Studies. There are studies on the adverse effects of acrylonitrile in humans. These studies link acrylonitrile exposure and lung cancer. It has also been suggested that acrylonitrile may have the potential to cause prostate cancer. Many of the studies have major limitations including insufficient quantification of exposure, short follow-up, small study population, and inadequate evaluation of confounding associations. Additional studies would be useful in clarifying the cancer risk and estimating the exposure levels that lead to these effects. 

After adjusting for the confounding factors of age, sex, nutritional status, and season, harbor porpoises (Phocoena phocoena) that died from disease had higher levels of PCBs than victims of trauma [80], providing a more rigorous evaluation of the link between contaminants, the immune system and disease outcome in marine mammals. Such studies can yield insight into the real world of immunotoxicity among marine... 

Most studies on spinal reflexes have utilized spinally transected animals. This preparation allows for the evaluation of drug actions on spinal postsynaptic 5-HT receptors without the potential confounding influences of tonic descending 5-HT activity or the influence of supraspinal 5-HT neuronal or receptor activity. 

Comparison of the relative sediment toxicity of different SPs can be difficult as there are a variety of different test methods and endpoints evaluated, in addition to other confounding factors relating to sediment quality. Amweg et al. [28] determined the toxicity of six SPs to //. azteca in 10-day studies at 23 °C in natural sediments containing 1-6% OC. Toxicity data were reported as bulk sediment concentrations and normalized to the organic carbon content (Table 5). The results indicated that normalization removed some, but not all, of the variability between sediments. Other factors such as sediment texture may also affect bioavailability and hence apparent toxicity in sediment studies. 

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. 

Behavioral Evaluation. Behavioral assessment of nonrodents is generally more difficult than evaluation of rodents because of their larger size, difficulties associated with handling and manipulation, and their greater awareness of and reactivity to the experimenter. Such factors can confound detection and/or interpretation of more subtle test compound-related behavioral changes. 

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