In order to build knowledge and theories, the scientific method requires that we either integrate earlier findings with new or alternatively that we challenge previous knowledge if we find that new findings contradict the old. There is a tacit understanding in science that “once is not enough” which implies that experiments should be replicated in order to verify findings. Popper wrote that we do not take our observations seriously or treat them as scientific until we have reproduced and tested them [1]. The reason why description of the methods used in research papers is so important is that it should be possible for other researchers to set up a similar experiment to check the viability of previous results. Despite this, it is rare that replication studies are carried out.
As long ago as 1995, researchers drew attention to the fact that studies regarding incidence and prevalence of mastitis were lacking, in part due to the problem of collecting trustworthy data [2]. These problems remained unsolved ten years later [3]. Despite this, the incidence of lactational mastitis is cited in almost all studies published on the subject.
Incidence
Incidence reports the number of new cases of a disease in a defined population during a specific time [4]. The most commonly used time period is one year but this is not a strict rule. The incidence rate is the number of new cases divided by the number in the population at risk. Therefore, in order to report the incidence rate of mastitis in breastfeeding women, it is first necessary to know the size of the population at risk, that is the population of breastfeeding women. Although most countries can give a reasonable estimation of the percentage of women who breastfeed their babies, numbers of women currently breastfeeding is a statistic that is not readily available since the population under study is in constant change by virtue of the fact that as some mothers cease breastfeeding their babies others have just given birth and are initiating breastfeeding.
Incidence rates in the literature
Reported figures vary greatly between countries and continents and may also differ over time. It is not uncommon for reports of mastitis incidence to be based on a breastfeeding cohort, which is assumed to be the population at risk. This can be problematical since a study cohort may have been selected for a specific research purpose and may not necessarily be a representative sample of all breastfeeding women in the study’s uptake area.
Many Australian studies have considered the incidence rate of mastitis; in 1995 researchers reported an incidence rate of 4.9%, which was based on an estimation of the number of women likely to be breastfeeding at three months postpartum [2]. Another Australian research group based their estimation of a 20% incidence on a cohort of 1075 postpartum women, but the number “at risk” was not considered [5]. A more recent Australian study of 1193 women estimated an incidence density using the number of completed weeks of women breastfeeding in four-week blocks and found a cumulative incidence in the first six breastfeeding months of 17.3% [6]. In the USA researchers followed a cohort of 840 women to 12 weeks postpartum and calculated that within the cohort, the overall incidence rate was 8.1% [7]. New Zealand researchers followed a cohort of 350 women up to one year postpartum and an incidence rate of 23.7% was reported [8]. It is unclear whether the population at risk was all mothers who had continued to breastfeed to one year postpartum. A cumulative incidence as high as 27% has also been reported from Australia [9].
In Europe, a Swedish study showed a mastitis incidence of 6%, which was based on breastfeeding statistics that allowed an estimate of the population at risk, which was considered as all breastfeeding women in the study’s up-take area during the time of data collection [10].
An early American study by Riordan and Nichols reported an incidence of 33% in 1990, in a descriptive study of lactational mastitis in long-term breastfeeding women and this is the single highest incidence reported in the scientific literature [11]. The population in Riordan and Nichols study comprised of three groups of participants at conferences sponsored by the International Lactation Consultant Association (ILCA) and La Leche League International (LLLI). Women were eligible for participation if they had ever breastfed. One of the questions that the study aimed to answer was “What is the incidence of mastitis in long-term breastfeeding women”?
The population at risk was seen in Riordan and Nichols’ study as all the women who were attending the conferences and who had ever breastfed. The cases of mastitis that were analysed were cases that the study population had experienced during breastfeeding of their children. For some of the analyses in their study, the authors included only those reporting breastfeeding during the previous five years. However, for the analysis of incidence, the whole study population was used. This is a statistic that is difficult to extract from the text but since the authors wrote that one-third of the women reported having mastitis with their last breastfed child and this number was 60, it appears that the population on which the calculation of incidence was based must have been 180.
Since the incidence of mastitis in the Riordan and Nichols’ study is the highest incidence ever reported, and is commonly cited [5, 7, 8, 10], it was of interest to attempt to replicate their investigation and to compare findings.
The aim of this study was to collect data regarding incidence and experiences of lactational mastitis from women attending a meeting of lactation specialists and to compare findings in a similar population to that reported in 1990 by Riordan and Nichols.