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Incidence and Prevalence
Incidence and Prevalence
Incidence: measures the occurrence of new cases in the population at risk
Cumulative incidence: proportion of new cases in the population at risk over a period of time
Incidence rate: rate of new cases per person-time at risk
Prevalence: proportion of existing cases in the total population
Point prevalence: prevalence at a single point in time
Period prevalence: prevalence during a period of time
Increasing Incidence and Prevalence of Acquired Hemolytic Anemias in Denmark, 1980–2016
Increasing Incidence and Prevalence of Acquired Hemolytic Anemias in Denmark, 1980–2016
In this study, researchers evaluated the incidence and prevalence of rare acquired hemolytic disorders, which include autoimmune hemolytic anemia, cold agglutinin disease, paroxysmal nocturnal hemoglobinuria, drug-induced hemolysis, and acquired hemolysis not otherwise specified. To do so, they analyzed the Danish National Patient Register for all those diagnosed with one of these disorders from 1977 to 2016 (5848 patients total.) When combined with demographic data from the Danish Civil Registration System, they were able to determine the incidence and prevalence of different acquired hemolytic disorders based on age and sex. When comparing the incidence rates from 1980-1993 and 2008-2016, they noticed that the incidence rates of the latter time period were markedly higher for each disorder except drug-induced hemolysis. In addition, the prevalence proportions from 1980 to 2015 also increased for all disorders. This marked increase could be in part due to the increased amount of clinical screening, but also this knowledge should be used to plan for healthcare in the future.
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7250289/
Comparing Disease Frequencies
Comparing Disease Frequencies
Absolute measures of comparison: measures the difference between two measures (risk difference)
RD < 0 if the exposure leads to a decreased risk of an outcome
RD = 0 if there's no association
RD > 0 if the exposure leads to an increased risk of an outcome
Relative measures of comparison: measures the ratio between two measures (risk ratio)
RR < 1 if the exposure leads to a decreased risk of an outcome
RR = 1 if there's no association
RR > 1 if the exposure leads to an increased risk of an outcome
A 2x2 table is often used to organize this data
Fluid Overload and Mortality in Adult Critical Care Patients—A Systematic Review and Meta-Analysis of Observational Studies
Fluid Overload and Mortality in Adult Critical Care Patients—A Systematic Review and Meta-Analysis of Observational Studies
In this systematic review, Messmer et. al. performed a meta-analysis focusing on studies regarding patients who have received large amounts of fluid, thus leading to fluid overload, and seeing if there was an association with mortality. Thirty-four studies, with 31,076 patients, were included in this study. They found that the adjusted relative risk for fluid overload and cumulative fluid balance were 2.79 and 1.39, respectively. The adjusted relative risk for fluid overload and cumulative fluid balance was increased when considering patients who had been in the ICU for three days. When considering the amount increase of fluid, every liter increase in fluids corresponded to a risk of mortality 1.19 times higher. Thus, the researchers concluded that fluid overload and increased cumulative fluid balances have a positive association with mortality.
The data used for the 2x2 table below comes from one of the studies used in the meta-analysis: Fluid overload is associated with an increased risk for 90-day mortality in critically ill patients with renal replacement therapy: data from the prospective FINNAKI study. Vaara et. al. examined a population of 296 patients who had been treated with renal replacement therapy (RRT) for acute kidney injury. After eliminating 13 patients with incomplete data on fluid balance, they determined the number of people with/without fluid overload as well as the 90-day mortality rate for each group.
The mortality rate of those with fluid overload is 45/76 = 59.2%. Out of those with fluid overload, 592 out of 1,000 people died.
The mortality rate of those without fluid overload is 65/207 = 31.4%. Out of those without fluid overload, 314 out of 1,000 people died.
The absolute mortality rate difference is 278 cases per 1,000 people. Compared to those without fluid overload, those with fluid overload had an increased risk of 278 per 1,000 people.
The relative mortality rate ratio is 1.885. Patients with fluid overload were 1.885 times as likely to not survive as those without fluid overload.
Question: What would the excess relative risk be?
Sources:
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