lipid metabolism, and accumulation of uremic toxins [2]. Volume overload and poorly controlled hypertension ultimately lead to left ventricular hypertrophy, cardiomyocyte, and heart failure [3]. In patients with chronic kidney disease, cardiomyocyte was associated with increased cardiovascular mortality and rapid renal progression [4]. Moreover, similar results have been reported by [PERSON] et al. in their work on HD patients [5]. They concluded that HD patients with cardiomegaly were at higher risk for cardiovascular events and all-cause mortality.  
Cardiomegaly refers to cardiac enlargement and is defined when the cardiothoracic ratio (CTR) is greater than or equal to 50% [6]. The CTR is the ratio of the transverse diameter of the heart outline to the transverse diameter of the chest. Among various clinical image studies, the measurement of CTR on a posterior-anterior projection of a chest X-ray is simple, cost-effective, and routinely applied in dialysis units for the assessment of cardiac size and volume status [7]. Undiagnosed or uncontrolled cardiomegaly eventually leads to decompensated heart failure, malignant ventricular arrhythmias, and sudden cardiac death [6; 8]. A higher CTR is also related to an increased risk for cardiovascular death in patients with end-stage kidney disease (ESKD) [4; 5; 9; 10; 11]. Therefore, the regular follow-up of CTR in ESKD patients is important.  
The increased serum osmolality is an independent risk factor for developing ESKD and is also accurate for assessing fluid status [12; 13]. By detecting the sodium (chloride and bicarbonate), glucose, and blood urea nitrogen in the serum, serum osmolality is the main physiologic signal for regulating water balance in the body and is a gold standard for diagnosing dehydration [14; 15; 16; 17; 18]. A higher CTR is accompanied by hypoyosmolality due to the fluid overload, which is related to decreased excretion of arginine vasopressin [19; 20]. However, taking blood by venipuncture for osmolality detection should be decreased in the frequency in HD patients [12; 21; 22; 23]. Therefore, the development of an easy-to-perform and non-invasive method is crucial for evaluating the volume status in dialysis patients.  
Saliva, a kind of body fluid, can be obtained and assessed more easily. There is a growing interest in the application of salivary biomarkers for clinical diagnosis. Saliva contains a variety of organic and inorganic materials, such as hormones, cytokines, enzymes, antibodies, pathogens, metabolites, and electrolytes, etc. [24]. The complexity of salivary bio-components makes it possible to reflect the systemic conditions. Recent literature has demonstrated the diagnostic utility of saliva in varied systemic diseases, including insulin resistance and obesity [25], neuropsychiatric disorders [26], stroke [27], and chronic kidney disease [28], etc. Moreover, the composition of saliva is approximately 99.5 percent water. The osmolality of saliva is determined by the major ions like sodium, potassium, chloride, and bicarbonate, and is correlated with salivary conductivity [29; 30]. Both salivary osmolality and conductivity increased during progressive dehydration which was reversed after rehydration [30; 31; 32; 33]. We recently developed a portable, and non-invasive biodevice to measure salivary conductivity by electric signals of saliva samples [30]. We found there was a positive correlation between serum osmolality and salivary conductivity in normal populations. This biodevice can also effectively discriminate volumetric states among healthy adults, but its utility is unclear in ESKD patients. The aim of this investigation is to validate the correlation between salivary conductivity and serum osmolality in HD patients, and also to evaluate the association between salivary conductivity and fluid state, which was determined by CTR.