Seventy-two GPs participated in the study (), with 1448 patients eligible for analysis. According to these GPs, 218 (15%) asked for an antibiotic and 500 (35%) were prescribed an antibiotic ().
The prescription of antibiotics was associated with medical and non-medical information. Antibiotics were prescribed more often for older patients, in cases of a medical history of chronic obstructive pulmonary disease (COPD) or a patient reporting a feeling of sickness, in cases of prolonged coughing or symptoms like sputum, fever and shortness of breath, and in cases of more abnormal auscultatory findings or signs such as percussion dullness (). There was also a higher incidence of antibiotic prescription among the GPs with the highest number of patient encounters and home visits on a weekly basis, and GPs prescribing relatively more antimicrobials for systemic use (ATC J) in cost and in volume (ATC J cost ratio and ATC J volume ratio, respectively). Likewise, there was a strong link between perceived patient demand for an antibiotic and the prescription of one (odds ratio [OR] = 4.64, 95% confidence interval [CI] = 2.96 to 7.26).
For these associations, the dependence of a pair of responses belonging to the same cluster was highly significant, with the intra-cluster correlation coefficient being 0.20 on average.
Interaction terms that were not statistically significant or prevented the model from converging were not retained in the model. Of the variables dropped for the latter reason, only the GPs' year of birth, the number of patient encounters per week and ATC J cost ratio were significant effect modifiers of the univariate relationship between perceived patient demand for antibiotics and the prescription of antibiotics. For younger GPs (year of birth 1965 versus 1945), GPs with fewer patient encounters per week (80 versus 150) and GPs with a higher ATC J cost ratio (0.20 versus 0.10), patient demand was even more strongly associated with antibiotic prescription.
This resulted in a model containing seven interaction terms (patient age, smoking, number of abnormal auscultatory findings, GPs’ university of graduation, part-time working status, registration group [control versus intervention] and registration year [2000 versus 2001]) (P<0.05). After eliminating interaction terms with a P-value greater than 0.01, only one interaction term was retained in the model. In order to obtain a comprehensible and valid estimate of the effect of perceived patient demand for antibiotics on GPs’ antibiotic prescribing, the final model controls for this interaction term and all possible confounders ().
This model fits the data well (Horton goodness-of-fit [GOFHorton]31 P
= 0.71; GOFHosmer–Lemeshow32 P
= 0.72). For the patients in the final models (n
= 819), the univariate association between perceived patient demand for antibiotics and the prescription of an antibiotic (OR = 4.60, 95% CI = 2.59 to 8.17) was very similar to that for all patients (n
Because of the introduction of GPs' characteristics in this model, the dependence for a pair of responses belonging to the same cluster was no longer significant, the intra-cluster correlation coefficient being 0.02.
From the final model we learn that perceived patient demand for antibiotics is still significantly associated with the prescription of antibiotics. This association is independent of the other information the GPs recorded and of their characteristics (). Significant confounders of this association are aspiration risk (adjusted OR = 14.79, 95% CI = 1.24 to 176.71), an impression of sickness (adjusted OR = 2.28, 95% CI = 1.47 to 3.53), the presence of sputum (adjusted OR = 2.52, 95% CI = 1.62 to 3.92), the presence of of a headache (adjusted OR = 1.57, 95% CI = 1.03 to 2.37), thoracic pain (adjusted OR = 1.68, 95% CI = 1.07 to 2.65), the number of abnormal auscultatory findings (adjusted OR = 3.04, 95% CI = 2.03 to 5.54), investigating infection parameters (adjusted OR = 28.04, 95% CI = 2.86 to 274.54), patient referral (adjusted OR = 0.04, 95% CI = 0.01 to 0.14), the age of the GPs at the start of this study (adjusted OR of year of birth = 1.22, 95% CI = 1.01 to 1.47), previous professional training (adjusted OR = 0.20, 95% CI = 0.10 to 0.42]), having a spirometer in the practice (adjusted OR = 0.44, 95% CI = 0.21 to 0.90), and the practice location ().
The relationship between the effect of perceived patient demand on antibiotic prescribing for acute cough and the significant confounders of this relation. Adjusted odds ratios and 95% confidence limits.
However, the effect of perceived patient demand for an antibiotic on GPs' antibiotic prescribing depends on the outcome of the lung auscultation. When patient demand for antibiotics is perceived, patients are prescribed an antibiotic significantly more often when the lung auscultation is normal (adjusted OR = 20.83, 95% CI = 8.86 to 48.99]) (), or in case of only one abnormal auscultatory finding (adjusted OR = 4.79, 95% CI = 2.16 to 10.60]).
In case of a normal lung auscultation, the adjusted predicted probability for an antibiotic prescription is 0.09 (95% CI = 0.02 to 0.30) if no patient demand for antibiotics is perceived, compared to 0.84 (95% CI = 0.52 to 0.97) if patient demand is perceived. If only one abnormal auscultatory finding is present these probabilities are 0.16 (95% CI = 0.03 to 0.53) and 0.98 (95% CI = 0.92 to 1.00), respectively. If there is more than one abnormal auscultatory finding there is no relevant and significant difference between perceived patient demand and no perceived patient demand.