The need for controls in case control studies.

Abstract

Dear Editor, This is regarding the article entitled “Relationship between type 2 diabetic retinopathy and periodontal disease in Iranian Adults” published in North American Journal of Medical Sciences 2014; 6:139-44.[1] As per the authors, the study was designed to evaluate the frequency of periodontal disease in patients with type 2 diabetes mellitus (DM) and how this was related with the presence of diabetic retinopathy (DR). The authors compared periodontal parameters (plaque index (PI), community periodontal index of treatment needs (CPITN), periodontal disease severity measured in quartiles of probing depth (PD), and clinical attachment loss (CAL)) in a group of diabetic patients with retinopathy (n = 84) versus a group of diabetic patients without retinopathy (n = 129). Further, the authors picked up 73 age-and sex-matched individuals to serve as the control group. The authors need to be appreciated for their effort. However, I have a few concerns with this study. Effectively, the authors have created three study groups: Diabetics without retinopathy, Diabetics with retinopathy, and Control group. The authors, however, fail to make us understand as to which group serves as a control to which study group. A look at the results section makes this clearer. The authors state that the severity of periodontal disease was significantly correlated with the severity of DR (P < 0.011), and the risk of PDR was significantly higher in the presence of periodontal disease odds ratio (OR) = 2.80, P < 0.029). Therefore, the question arises as to what was the use of creating three groups in the first place if the corelation was to be obtained with regard to severity only. Was creation of a control group necessary? Importantly, there seems to be a mismatch in the numbers presented in the paper. The numbers seem confusing. The numbers in Table 1 do not match with numbers in Table 2. In Table 1, patients with DM with DR have been shown as 79 and without DR as 108, whereas in Table 2 they have been shown as 84 and 129, respectively. Further, the numbers with regard to distribution of patients in different groups as per age/sex and presence/absence of hypertension do not match. In control group [Table 1], the authors write male (M)/ female (F) as 45/69 that does not add up to 73. Similarly, for hypertension (no/yes) 51/58 does not add up to 73. There are similar errors in other groups as well. Table 1 Characteristics of study population Table 2 Periodontal disease and diabetic retinopathy