Respond to Vitamin D status in adult critically ill patients in Eastern India: An observational retrospective study.

Sir,

With respect to the article titled “Vitamin D status in adult critically ill patients in Eastern India: An observational retrospective study”[1] we would like to make the following comments.

Our study focused on critically ill patients rather than the general population

There are limitations in carrying out randomized controlled trials (RCTs) in critically ill patients, hence observational studies are becoming more popular to investigate the relationship between exposures, such as risk factors and outcomes, such as mortality and morbidity. RCTs are not always indicated or ethical to conduct in critically ill. Instead, observational studies are an important category of study designs. Well-designed observational studies have been shown to provide results similar to RCTs. Cohort studies and case-control studies are two primary types of observational studies. WH Frost, an epidemiologist from the early 1900s, was the first to use the word “cohort” in his 1935 publication assessing age-specific mortality rates and tuberculosis.[2] The modern epidemiological definition of the word now means a group of people with defined characteristics who are followed up to determine incidence of, or mortality from, some specific risk factors, all causes of death, or some other outcome[1]

Our cohorts were matched with well-validated statistical analysis. The discriminative powers of admission and lowest 25(OH) D values regarding day-30 mortality were evaluated by producing receiver operating curves (ROC). Binary end points were analyzed using Fisher's exact test. Continuous variables were compared using unpaired t-tests, Welch's tests, or Wilcoxon ranksum tests, as appropriate. All odds ratios and their corresponding 95% confidence intervals were calculated according to the profile-likelihood method. The time from inclusion to death in the two groups was compared by using the log-rank test, and the results were presented as Kaplan-Meier curves. Hazard ratios for death from vitamin D deficiency were calculated by logistic regression model. All P values were 2-tailed, and P < 0.05 were considered statistically significant.

• The disadvantages of case control observational studies are[3]

They are inefficient for rare exposures

Information on exposure is subject to observation bias

They generally do not allow calculation of incidence (absolute risk)

They are subject to selection bias.

We would like to thank the authors for their suggestions at the same time we would like to emphasize that in India where recourses are limited taking up large-scale double blind case control RCTs are not always practicable but at the same time we should always try with available recourses to present emerging evidences in clinical practice.