Diseases frequency

A pressing challenge for epidemiologists interested in occupational health is to derive an accurate picture of disease frequency. This challenge is met by two broad types of measurement prevalence and incidence. These arc briefly described below. 

The possibility for such studies will depend on the disease, disease frequency and whether the prescribing setting is in primary or secondary care. The value of these studies is likely to be greatest if data are generated as soon as possible after launch, and plans for implementation must occur well in advance of submission of the regulatory dossier. Such studies might also be a condition of registration. 

Secondly, the authors should have taken account in the study design and analysis of other variables that can influence the risk of disease and may have been related to the exposure of interest. Potential confounding by such variables should have been dealt with either in the design of the study, such as by matching, or in the analysis, by statistical adjustment. In cohort studies, comparisons with local rates of disease may be more appropriate than those with national rates. Internal comparisons of disease frequency among individuals at different levels of exposure should also have been made in the study. 

Providing adequate diagnostic accuracy, that is diagnostic sensitivity and diagnostic specificity, determined by the desired positive and negative predictive values which depend on disease frequency [124]. 

Ravnskov, U. 1992. Cholesterol lowering trials in coronary heart disease frequency of citation and outcome. Br. Med J. 305, 15-19. 

Common hydrogeological characteristics of endemic foci [12] and inverse relationship between altitude of wells and disease frequency in a longitudinal (cohort) study [63], pointed to potable water as a vehicle of the agent(s). However, none of the already mentioned or several dozen other non-organic substances were associated with the disease [64]. 

Wilson s disease is an autosomal recessive disorder of copper metabolism (see Chapters 20 and 30). It has a gene frequency of 1 in 200 and a disease frequency of 1 in 30,000. It is due to one of more than 200 mutations in a gene on chromosome 13 coding for a copper transporting ATPase... 

Chronologically, lead poisoning was first offered as an explanation for the occurrence of Balkan nephropathy [17-19]. The idea on lead-contaminated flour led to abandonment of water mills in a part of Central Serbia. This energetic public health action had no impact on the disease frequency. 

Recently, medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, j the cause of Lofata Bume s problems, has emerged as one of the most — common of the inborn errors of metabolism, with a carrier frequency ranging from 1 in 40 in northern European populations to less than 1 in 100 in Asians. Overall, the predicted disease frequency for MCAD deficiency is 1 in 15,000 persons. 

KJ Rothman, S Greenland. Measures of disease frequency. In KJ Rothman, S Greenland, eds. Modern Epidemiology. 2nd ed. Philadelphia Lippincott, WUhams Wilkins, 1998, pp. 29-46. 

Measures of disease frequencies include prevalence , which is the amount of disease that is already present in a population incidence , which refers to the number of new cases of contact dermatitis during a defined period in a specified population and incidence rate (IR), which is the number of non-diseased persons who become diseased within a certain period of time, divided by the number of person-years in the population. 

All measures of disease frequency consist of the number of cases as the numerator, and the size of the population under study as the denominator. As mentioned above, in case series the sample represented by the numerator is often biased. With few exceptions, however, the size of the denominator is unknown in many publications presenting the frequencies of OCD. Therefore it is not possible to calculate rates. This is the reason why studies among patient populations from dermatology clinics are not adequate for estimating prevalence or IRs. 

Another aspect that is important in health data interpretation is the actual data presentation, which should aim to summarize and organize data as accurately as possible. Visual displays (as in Figure 11.3) are often more comprehensible, allowing the easier identification of disease frequencies, tendencies, and comparisons between groups as well as other potential connections in the data. 

Epidemiological studies. Epidemiology is the study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems. Therefore, epidemiological studies harvest valid and precise information about the causes, prevention and treatments for disease. Types of epidemiological studies are experimental studies and observational studies (cohort and case-control studies). Homocysteine. Homocysteine is a sulfur-containing amino acid that occurs naturally in all humans. Elevated plasma homocysteine concentration is linked to an increased risk of ischemic stroke. Lowering homocysteine plasma levels is linked to increasing the intake of folic acid and vitamins Bg and Bi2-... 

Reliable estimates of the disease frequency apparently are lacking. As in the previously discussed sphingohpidoses the classical form of Gaucher s disease also was found to be particularly frequent in Jews. 

Where sufficient numbers of cases could be studied, single gene autosomal recessive inheritance could be demonstrated. Heterozygotes sometimes can be recognized. Our knowledge of the disease frequency in various populations is insufficient. Since the milky serum can easily be recognized, in this time of frequent routine blood studies the diagnosis should hardly be missed, once the physicians have become familiar with the disease. 

Labanowski M, Schmidt-Nowara W, Guilleminault C. Sleep and neuromuscular disease frequency of sleep-disordered breathing in a neuromuscular disease clinic population. Neurology 1996 47 1173-1180. 

When epidemiologists compare two human populations, one defined as being at risk and the other defined as the control, they begin by hypothesizing that there is no difference in disease frequency between the two populations. They then collect data to decide whether their hypothesis is correct or incorrect. The hypothesis of no difference between two populations is called the null hypothesis. The null hypothesis is accepted if it is decided that there is no difference between the two populations, and it is rejected if it is decided that there is a difference. There is a finite probability of committing an error and rejecting the null hypothesis when it should be accepted and of accepting the nnll hypothesis when it should be rejected. The decision to accept or reject the null hypothesis is associated with a specified level of statistical confidence in the data. For example, if the null hypothesis is rejected at the 0.95 confidence level, there is a 95% chance that the decision is correct (i.e., that there really is a difference between the study and control populations) and a 5% chance that the decision to reject the null hypothesis is erroneous (i.e., that there really is no difference between the two populations). 

Despite the power of well-designed epidemiological studies to detect differences in disease frequencies in human populations, it is important to keep in mind that statistical significance is not the same as toxicological significance. To make a convincing connection between chemical exposure and disease, one or more additional kinds of evidence are needed. First, there must be evidence that the at-risk population was actually exposed to the chemical of concern. Second, the chemical should obey the dose-effect relationship, i.e., the incidence of the toxic effect of concern should, if possible, be shown to increase as a function of exposure, either in the... 

There are two basic approaches to designing an epidemiological study to investigate accidentally induced chemical disease cohort and case control. In a cohort study, the at-risk population is defined as one that is known to have been exposed to a chemical of concern, and the frequency of disease is compared with that of a control population that has not been exposed. More than one disease may be tracked in a cohort study, depending on the known or suspected toxicides of the chemical of concern. Elevation of disease frequency, i.e., rejection of the null hypothesis at an acceptable level of statistical confidence, constitutes presumptive evidence of a causal link between exposure and disease. The causal connection can be strengthened by evidence of a dose-effect relationship, as discussed in Section 4.5. 

In a case-control study design, the point of departine is an at-risk popnlation that is defined, not as one that is known to have been exposed, but as one that is suspected of having an elevated disease frequency (the case). The disease frequency is compared with that of a sample drawn from a second population that is thought not to be at risk (the eontrol). Elevation of the disease frequency in the study group drawn from the case population compared with the study group drawn from the control ... 

Figure 5.1 shows that all three graphs used to characterize the relationship between the dose of toxic chemical and the frequency of toxic effect—i.e., the frequency histogram, the cumulative dose-effect curve, and the linear probit plot—put the logarithm of the dose on the x-axis. The reason the probit plot is linear is because of a clever invention a statistically derived scale that represents cumulative-effect frequency on the y-axis. Called probit units, or simply probits, this scale is based on a particular statistic, the standard deviation of the mean. Standard deviations correspond to fixed percentages of a population, and they can therefore be used in place of percentages to represent the fraction of a population that manifests a toxic effect. The mean dose corresponds to a standard deviation of zero because it is located in the exact middle of the bell curve. In terms of standard deviations, a disease frequency of 50% of the population is equivalent to zero standard deviations. One standard deviation below and above the mean corresponds, respectively, to manifestation... 

---