EUROPEAN PRESSURE ULCER ADVISORY PANEL

Prevalence and Incidence are both measures of disease frequency. While both have been used to record the number of people with pressure ulcers, they provide different perspectives on the scale of the problem.¹

Prevalence

Prevalence is defined as a cross-sectional count of the number of cases at a specific point in time, or the number of persons with pressure ulcers who exist in a patient population at a given point in time (see Table 1).²

Table 1: Prevalence

Pressure Ulcer Point Prevalence =
Number of persons with a pressure ulcer x 100
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Number of persons in a population at a particular point in time

Pressure Ulcer Period Prevalence =
Number of persons with a pressure ulcer x 100
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Number of persons in a population during a particular period in time

Incidence

Incidence is defined as the number of persons who develop a new pressure ulcer at an initially pressure ulcer free location, within a particular time period in a particular population. Several approaches to measuring incidence have been explored (see Table 2).

Pressure Ulcer Cumulative Incidence =
Number of persons developing new pressure ulcers at an initially pressure ulcer free location x 100
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Number of persons (with or without pressure ulcers) in population at beginning of time period

Pressure Ulcer Incidence Density =
Incidence Rate =
Number of persons developing new pressure ulcers at an initially pressure ulcer free location
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Total person days* free of new pressure ulcers

Table 2: Incidence (* Sum of all the days over which each patient participated in the study)

Characteristics of Prevalence and Incidence

As mentioned earlier, prevalence and incidence are different measures of disease frequency. The characteristics of prevalence and incidence are summarized in Table 3 below.

Purpose

Before deciding to measure either pressure ulcer prevalence or incidence it is useful to consider the information the different measures can provide.

PREVALENCE measures the number of patients with pressure ulcers at a certain point or period in time. Thus, it provides an institution with insight into the magnitude of the problem of pressure ulcers at a given point in time, and may be an aid in planning for health resources and facilities. For example, during a prevalence survey it is possible to record how many devices (e.g., alternating mattresses) are being used at that specific moment.

Given that many prevalence surveys also collect information upon aspects of prevention and treatment, such surveys may allow inferences to be made regarding the compliance with prevention and treatment protocols at that specific moment.

INCIDENCE measures the number of persons developing new pressure ulcers during a period in time and thereby provides insight into the nature of patient groups who are at risk of pressure ulcer development. Furthermore, incidence may allow inferences to be made regarding the effectiveness of preventive measures and the compliance with prevention and treatment protocols. Also, incidence provides insight into the magnitude of the problem of pressure ulcers developed within the current care provider.

The interpretation of prevalence and incidence date can be challenging

When interpreting particular prevalence or incidence data it is important to understand the factors that may influence the apparent size of the pressure ulcer population.

PREVALENCE will be affected by the number of persons with a pressure ulcer present at admission to the current care provider. If this number is high, prevalence proportions may be high too. For example, where patients with pressure ulcers are referred to a specific institution, for example because of the expertise of the institution in pressure ulcer treatment, this admission practice will influence the prevalence of pressure ulcers. Prevalence is also influenced by discharge practices. For example, a hospital that is able to quickly discharge patients with a pressure ulcer to a nursing home may have a lower prevalence of pressure ulcers than a hospital that can only discharge patients after the pressure ulcer has healed.

If the prevention and treatment protocols are of low quality or compliance with these protocols is low, then it is likely that both the prevention and treatment of pressure ulcers will be sub-optimal. This may lead to patients experiencing their pressure ulcers for a longer period of time. These patients will then be more likely to be identified during a prevalence survey and hence prevalence may be high.

INCIDENCE is affected by discharge practices, given that this rate will be influenced by the length of stay of each patient within the care provider. For example, a hospital that discharges patients within a few days, i.e., before pressure ulcers have a chance to develop, is likely to have a lower incidence than a hospital that admits patients for a longer period of time. It is generally assumed (although unproven) that pressure damage may first appear three to five days after the insult to the skin and soft tissues occurred. In patients with a length of stay of, for example only three days, pressure damage may have occurred but not yet be visible. These pressure ulcers would not be registered, resulting in a lower incidence rate.

If the prevention protocol is of low quality or the compliance with these protocols is poor, then preventive care is not optimal and therefore incidence may be higher. As patients with an existing ulcer but who develop additional pressure ulcers may be included in incidence studies, then adherence to treatment protocols may also influence incidence. Where a pressure ulcer heals quickly due to staff compliance with a high quality treatment protocol, it is possible that a pressure ulcer may then re-occur on the previously injured site and this may then be counted as a 'new' ulcer. This illustrates the complexity of determining which pressure ulcers, and which patients, to include in any incidence monitoring project.

Both prevalence and incidence are influenced by the case mix of the institution. While variations may arise it is likely than where two institutions provide identical preventative care then the centre with more patients at high risk of developing pressure ulcers may have both a higher incidence of pressure ulcers. In the previous example the prevalence of pressure ulcers within the centre with most high risk patients may also be higher but this indicator will be susceptible to the admission of patients with pre-existing pressure ulcers.

Practical issues related to the collection of prevalence and incidence data

Measuring incidence rates requires a longitudinal design and in consequence such studies are likely to be more labour intensive, and hence costly than would be a point prevalence survey. The costs of both prevalence and incidence monitoring may be reduced if patient medical and nursing records are held electronically with appropriate fields available for the recording of pressure ulcers. The frequency of patient observation to record incidence of new pressure ulcers may depend upon care setting, but it is likely that in acute care daily observation of the skin would be required. Regardless of whether incidence or prevalence is to be recorded the accuracy of the submitted data needs to be assessed.

Despite the fact that many studies have been performed in various countries to record incidence and (primarily) prevalence, comparison between this data are extremely restricted given factors such as the use of different pressure ulcer classification systems, incomparable patient groups, small samples and differences in data sources.2-4 Therefore, data must always be examined in light of the specific study methodology.2 Appendix 1 gives some practical suggestions for measuring prevalence and incidence.

The selection of either prevalence or incidence data as a means of illustrating the occurrence of pressure ulcers should be made following a detailed consideration of the strengths and limitations of both epidemiological measures.

The European Pressure Ulcer Advisory Panel considers that measuring pressure ulcer incidence is the most appropriate approach if the goal is to understand how the introduction of new protocols and interventions has affected the number of patients with pressure ulcers. Where the goal is to identify the current size and characteristics of the pressure ulcer affected population, then prevalence may be more appropriate. However, the costs associated with the implementation of an incidence monitoring scheme may be prohibitive, and for this reason prevalence may be selected even though it may not be fully appropriate.

Reference list

1. Rothman K, Greenland S. Modern Epidemiology. 2nd Edition. Philadelphia: Lippincott Williams & Wilkins, 1998.
2. National Pressure Ulcer Advisory Panel. Pressure Ulcers in America: Prevalence, Incidence, and Implications for the Future. Cuddigan J, Ayello EA, Sussman C, editors. 2001. Reston, VA, NPUAP.
3. Allman RM. Pressure ulcer prevalence, incidence, risk factors, and impact. Clin Geriatr Med 1997; 13(3): 421-436.
4. van-Rijswijk L. Epidemiology. In: Morison M, editor. The Prevention and Treatment of Pressure Ulcers. London: Mosby, 2001: 7-15.

Appendix 1
Practical recommendations for measuring both pressure ulcer prevalence and incidence

1) Patient inclusion / exclusion

Clearly define the population to be surveyed before starting data collection. Be aware that exclusion of certain patients or groups of patients makes comparison with other studies more difficult (problem of generalization). Patients with pressure ulcers at the start of the study should not be excluded, even from incidence studies. They have a high risk of developing new pressure ulcers on other locations. However, only any new pressure ulcers should be counted when calculating incidence figures. Base your incidence or prevalence data upon the number of patients with, or developing pressure ulcers and not upon the number of pressure ulcers that develop.

2) Survey methodology - general issues

It is essential that the assessors are able to distinguish pressure ulcers from other types of wounds, for instance incontinence damage, to prevent misclassification. Therefore observers should be trained in the classification of pressure ulcers using the EPUAP pressure ulcer grading system. In research studies, inter-rater reliability should be formally checked and reported.

Inspect all of the pressure areas of each individual patient. Skin inspection is important for using medical or nursing records is often not a very reliable method of pressure ulcer data collection! Always remember that caregivers are not always aware of the existence of pressure ulcers. Use a transparent device for facilitating the assessment of grade I (non-blanchable erythema).

Assessing the skin of the surveyed patients should be carried out by two observers working independently; ideally one of the observers should not be a staff member of the unit where the patient is located.

Frequently the use of preventive interventions may be recorded during prevalence surveys; if these are to be recorded then note which devices are in place at the bedside or chair of each surveyed patient. Not all preventive measures used are reported in records while not all preventive measures mentioned in the medical and/or nursing records are used as prescribed!

Specific comments regarding incidence monitoring:

When considering acute care, it may be considered to be a general rule that the less frequently patients are observed, the less reliable the collected incidence data becomes. In acute care, patients should be assessed at least daily if non-blanchable erythema is used as one of the outcome measures. Where only more severe pressure ulcers (grades II and higher) are used as outcome measures then patients should be assessed at least every two or three days in acute care. In non-acute care the interval between skin assessments may be longer dependent upon the logistical aspects of visiting patients.

Reporting of pressure ulcer incidence and prevalence data

In all cases the population surveyed should be fully described in any report or publication; this facilitates comparison with other pressure ulcer epidemiological data. Among the items that may be described are patient ages, gender, vulnerability to developing pressure ulcers, mobility, activity, expected length of stay and care location (acute, non-acute, and specific populations such as intensive care).

Pressure ulcer incidence and prevalence data should be based upon the number of patients with pressure ulcers. If any individual has more than one pressure ulcer, that person is counted only once.

It may be useful to report data in two formats; the first including all pressure damage (including areas of non-broken skin), the second excluding Grade I pressure ulcers and so reporting only areas where the skin was broken.

Comparison of results (be they incidence or prevalence) between different care providers or within a single provider over time should be done with caution (if at all). In any comparison patient characteristics and case mix should be taken into account.

Specific issues related to the reporting of Incidence data

After the start of any incidence monitoring project, only 'new' pressure ulcers (pressure ulcers developed after the start of the study) should be recorded. New pressure ulcers may occur in patients who have pressure damage present before the start of the project; record these individuals the first time they develop a new pressure ulcer. It is possible that those pressure ulcers that develop during the first few days that a patient is being monitored were the result of excessive tissue loading prior to the entry into their current care location. This last statement is commonly believed although there is little evidence to support its hypothesis. Measuring incidence in non acute care may be compounded by the relatively low numbers of patients present at the start of the study, this challenge of reporting incidence is further complicated where recruitment to the population is slow. In such circumstances it may be more appropriate to report pressure ulcer prevalence data.