Calculating the sample size in scientific studies is one of the critical issues as regards the scientific contribution of the study. The sample size critically affects the hypothesis and the study design, and there is no straightforward way of calculating the effective sample size for reaching an accurate conclusion. Use of a statistically incorrect sample size may lead to inadequate results in both clinical and laboratory studies as well as resulting in time loss, cost, and ethical problems. This review holds two main aims. The first aim is to explain the importance of sample size and its relationship to effect size (ES) and statistical significance. The second aim is to assist researchers planning to perform sample size estimations by suggesting and elucidating available alternative software, guidelines and references that will serve different scientific purposes.

Summary: Background:25 (OH) vitamin D3 (25(OH)D) and parathyroid hormone (PTH) are important regulators of calcium homeostasis. The aim of this study was to retrospectively determine the cut–off for sufficient 25(OH)D in a four-season region and the influence of age, seasons, and gender on serum 25(OH)D and PTH levels.Methods:Laboratory results of 9890 female and 2723 male individuals aged 38.8±22.1 years who had simultaneous measurements of 25(OH)D and PTH were retrospectively analyzed by statistical softwares. Serum 25(OH)D and PTH levels were measured by a mass spectrometry method and by an electrochemiluminescence immunoassay, respectively.Results:Mean serum 25(OH)D levels showed a sinusoidal fluctuation throughout the year and were significantly (p<0.01) higher in summer and autumn. On the other hand, PTH levels were significantly higher (p<0.01) in women and showed an opposite response to seasonal effects relative to 25(OH)D. Lowest levels of 25(OH)D were detected in people aged between 20 and 40 years whereas PTH hormone levels were gradually increasing in response to aging. The significant exponential inverse relationship that was found between PTH and 25(OH)D (PTH=exp(4.12–0.064*sqrt(25(OH)D)) (r=−0.325, R–squared=0.105, p<0.001)) suggested that the cut–off for sufficient 25(OH)D should be 75 nmol/L.Conclusions:Our retrospective study based on large data set supports the suitability of the currently accepted clinical cut–off of 75 nmol/L for sufficient 25(OH)D. However, the issue of assessing Vitamin D deficiency remains difficult due to seasonal variations in serum 25(OH)D. Therefore, PTH measurements should complement 25(OH)D results for diagnosing Vitamin D deficiency. It is imperative that seasonally different criteria should be considered in future.

Background: It is known that inflammatory responses play an important role in the pathophysiology of COVID‐19.Aims: In this study, we aimed to examine the role of kynurenine (KYN) metabolism on the severity of COVID‐19 disease AQ5.Materials & Methods: Seventy COVID‐19 patients of varying severity and 30 controls were included in the study. In addition to the classical laboratory parameters, KYN, tryptophan (TRP), kynurenic acid (KYNA), 3 hydroxykynurenine (3OHKYN), quinolinic acid (QA), and picolinic acid (PA) were measured with mass spectrometry.Results: TRP, KYN, KYN:TRP ratio, KYNA, 3OHKYN, PA, and QA results were found to be significantly different in COVID‐19 patients (p < 0.001 for all). The KYN:TRP ratio and PA of severe COVID‐19 patients was statistically higher than that of mild‐moderate COVID‐19 patients (p < 0.001 for all). When results were examined, statistically significant correlations with KYN:TRP ratio, IL‐6, ferritin, and procalcitonin were only found in COVID‐19 patients. ROC analysis indicated that highest AUC values were obtained by KYN:TRP ratio and PA (0.751 vs 0.742). In determining the severity of COVID‐19 disease, the odd ratios (and confidence intervals) of KYN:TRP ratio and PA levels that were adjusted according to age, gender, and comorbidity were determined to be 1.44 (1.1–1.87, p = 0.008) and 1.06 (1.02–1.11, p = 0.006), respectively.Discussion & Conclusion: According to the results of this study, KYN metabolites play a role in the pathophysiology of COVID‐19, especially KYN:TRP ratio and PA could be markers for identification of severe COVID‐19 cases.

Vitamin D and intact parathyroid hormone (iPTH) play a crucial role in calcium homeostasis and bone health of children. Serum level of 25-hydroxyvitamin D (25-OHD) is considered to be the most accurate marker for vitamin D status. However, there have only been a few studies, with limited number of subjects, investigating the relationship between 25-OHD and parathyroid hormone (PTH) in children. The aim of this study was to evaluate the seasonal 25-OHD levels and its associations with intact parathyroid hormone (iPTH) in Turkish children at all pediatric ages; and then to define a critical decision threshold level for 25-OHD deficiency in Turkish children. A retrospective record review of 90,042 children, was performed on serum 25-OHD and for 3525 iPTH levels. They were measured by mass spectrometry method and by electrochemiluminescence immunoassay simultaneously. 25-OHD levels showed a sinusoidal fluctuation througout the year; being significantly higher in summer and autumn (p < 0,01). 25-OHD levels decreased with respect to age. The significant inverse relationship that was found between iPTH and 25-OHD suggests that the inflection point of serum 25-OHD level for maximal suppression of PTH is at 30 ng/ml. As the rate of vitamin D deficiency decreases in the early years due to vitamin D supplementation, the recommendation should be set due to a clinical threshold level of 30 ng/ml for 25-OHD based on PTH levels in children of our population.