Our analysis of 26 relevant studies showed that 43% of cancer patients have a negative PMI score: nearly one of two patients is undertreated. Such a percentage is exceedingly high, but a temporal trend suggests a slight improvement in cancer pain management throughout the years. It is likely that this condition comes from a situation in progress due to better medical education and greater attention paid by national and international agencies such as WHO [42
] and The Joint Commission [43
]. A geographical and economical trend emerged as well in favor of the United States and other rich countries. Wealthier health systems can sustain and encourage a better pain management through pain control campaigns and drug full covering by national health systems or health insurances. The multivariate analyses also showed an association between negative PMI and settings not specific for cancer patients, maybe due to a lack of specific education in pain management for physicians who have not specialized in oncology or palliative care.
Once the percentage of undertreated patients using a pain index was determined, several investigators tried to identify prognostic factors. Gender and advanced age do not seem to play a role consistently. Patients who were rated less ill (better Performance Status) and at an early stage of the disease (no distant metastasis) were more likely to receive inadequate analgesia. A possible explanation is that patients who look less ill may also be judged to have less pain [9
]. A different interpretation of this result is that metastatic patients are treated by a pain expert more frequently than patients at an earlier stage of disease. The discrepancy between the physician's and patient's estimate of the severity of pain experienced and the potential role played by education and ethnicity can suggest that a failure in physician–patient communication may also play a role in undertreatment genesis.
Although PMI is not accurate for prescribing drugs for an individual and not appropriate to evaluate quality of care at an individual level, it provides a rough estimate of how pain is treated in the population. However, it does not take into account other aspects of the complex problem of cancer pain management: the patient's compliance to the therapy [10
], the dosage [45
] and route of administration of the most potent analgesic prescribed, potential associations with further analgesic adjuvant drugs (i.e. antidepressants, anticonvulsants) and with other nonpharmacological therapies (i.e. acupuncture, biofeedback). Also, the index takes into account drugs recently prescribed but not yet taken, thus patients with severe pain who were prescribed morphine at the time of the survey are classified as adequately treated. Some authors developed alternative indexes [10
] just to incorporate in the score some of these additional aspects. When the PMI Cleeland, Ward and Zelman were compared by de Wit et al. [15
], the percentage of agreement was very high, especially for Cleeland and Ward (kappa from 0.81 to 1.00), which suggests a broad overlap and a common structure among these measures. The Amsterdam PMI, on the contrary, showed only a fair agreement with the three PMIs [12
], meaning that it may give an estimate of pain treatment adequacy different from the other three. Once these limitations are considered, PMI Cleeland can be used not to obtain a score of any aspect related to pain management, but to find out the consistency between the physician's order and good practice guidelines. The usefulness of this indicator is proved by the great number of studies that have used this score since 1994, and its application to medical conditions other than cancer, particularly for AIDS.
Our study recognizes some limitations: some shortcomings are related to the intrinsic characteristics of the instrument used, whereas others are related to the impossibility of excluding the existence of additional studies which used PMI that were not published or not retrievable through Medline with the search method used. Also, the attempt of identifying variables predictive of better pain management, carried out on a study level and not on an individual patient level, carries significant risk of low sensitivity.
In addition, the large variability of undertreatment prevalence across studies and settings maybe also related to some hidden (not measured) variables that could not be taken into account in our univariate and multivariate analyses because they were not assessed by original authors and thus not reported in the papers. This fact is suggested by the results from an ongoing prospective study carried out in Italy in 2007 [46
] where PMI Cleeland was prospectively utilized to assess the prevalence of undertreatment in a cohort of 1801 cancer patients seeking care in 110 Italian oncologic and palliative centers. Overall, the prevalence of PMI negative scores at the time of study inclusion was ~25%, with large variations according to several variables including patients, centers and settings characteristics, such as presence of bone metastasis, ongoing chemotherapy or adjuvant therapy and type of recruiting centers (oncologic or palliative). The case-mix of the cases recruited yielded a large variability across subgroups, reaching a prevalence of up to 45% in some subgroups.
In conclusion, PMI maybe useful in evaluating the quality of the analgesic care in large sample cases. The proportion of cancer patients whose pain is undertreated is still high, reaching almost a half of all the patients considered in this review. Variability of its occurrence in this sample of studies suggests important pain determinants: geographical area (Europe and Asia), countries with lower economic level and setting not specific for cancer care and management. These results are important for implementing policies to reduce inappropriate high pain prevalence and to address barriers to pain control in the neglected context.