Hydration Status in Geriatric Patients—Subjective Impression or Objective Parameter? The Hydr-Age-Study

<b>Background/Objectives:</b> Assessing the hydration status (HS) in geriatric patients remains challenging due to multimorbidity, polypharmacy, and cognitive impairment. Common indicators like reduced skin turgor and dry mucous membranes are unreliable. The Hydr-Age-Study is a prospective observational pilot study with a post hoc analysis to evaluate the diagnostic accuracy of clinical, laboratory, and instrumental methods to assess HS in hospitalised older adults. <b>Methods</b>: Upon admission, patients underwent an assessment including their medical history, a clinical evaluation, laboratory tests, ultrasound examination, and bioimpedance analysis. These data were collected and independently reviewed by two experts who diagnosed each patient's current HS. This diagnosis served as the clinical reference standard for evaluating the diagnostic accuracy of each method. <b>Results</b>: Twenty-six methods were examined, of which four achieved an AUC > 0.8. Axillary dryness showed a high diagnostic accuracy for hypohydration (AUC = 0.854), with a sensitivity of 83.3% and a specificity of 82.8%. Inferior vena cava (IVC) ultrasound effectively detected both hypo- and hyperhydration. A subxiphoid IVC diameter ≤ 1.95 cm identified hypohydration with 90.9% sensitivity and 50.6% specificity. For hyperhydration, a diameter of ≥2.15 cm provided strong diagnostic performance in both subxiphoid and transcostal views. <b>Conclusions</b>: Axillary dryness and IVC sonography demonstrated the highest diagnostic accuracy. No other methods exceeded an AUC of 0.80. In the absence of a gold standard, a structured clinical consensus provides a feasible and reproducible approach to establish a clinical reference standard. These findings may contribute to the development of a standardised assessment protocol in geriatric medicine.