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Marquina et al. J Cancer Metastasis Treat 2020;6:6  I  http://dx.doi.org/10.20517/2394-4722.2019.39                         Page 3 of 8

                          Table 2. Basic blood and urine tests for the diagnosis of the cause of hyponatremia
                          Blood test (serum)             Urine test             Gasometer
                          Protein
                          Glucose
                          Urea
                          Creatinine                     Creatinine
                          Osmolality                     Osmolality
                          Sodium                         Sodium                 Sodium
                          Potassium                      Potassium
                          Chlorine                       Chlorine
                          Cortisol
                          TSH
                          T4
                          TSH: thyroid stimulating hormone; T4: thyroxine


               Table 3. Approach to diagnosis
                Etiology approach for hyponatremia
                The basic hyponatremia approach is based on clinical history, physical examination, full blood test (as described in Table 2), timing of
                the onset of hyponatremia, symptoms, and type of hyponatremia [Table 4]
                Urine sodium: essential for the   With renal sodium loss (Urine sodium > 25 mmol/L): diuretics, bicarbonate intake, primary
                differential diagnosis of hypovolemic  adrenal insufficiency (Addison’s disease), isolated hypoaldosteronism, and salt wasting syndrome
                hyponatremia.            Without sodium renal loss (Urine sodium < 20 mmol/L): gastrointestinal losses (vomiting and
                                         diarrhea), burns, hemorrhage, and pancreatitis
                Urine osmolality: essential for the   ≤ 100 mOsm/kg: secretion of the ADH is inhibited, polydipsia with or without low solute intake,
                differential diagnosis of euvolemic   water intoxication, and administration of hypotonic fluids
                hyponatremia             > 100 mOsm/kg: Secretion of ADH is not inhibited, ACTH deficit, severe hypothyroidism, pain,
                                         postsurgical stress, nausea, vomiting, the syndrome of inappropriate antidiuretic hormone
                                         secretion, use of thiazides, etc.
               ADH: antidiuretic hormone; ACTH: adrenocorticotropic hormone

               out true hyponatremia. Pseudohyponatremia, induced by high protein or lipid levels, can be excluded by
               the determination of total blood sodium by gasometer.

               The best and most direct way to ascertain whether the patient presents hypovolemic or euvolemic
                                                                                                    [18]
               eunatremia is by neck inspection of the highest point of the internal jugular vein pulse [Table 1] . The
               evolution of serum creatinine together with SNa is also a good parameter. Serum creatinine usually
               increases when natremia drops in the hypovolemic patient and usually decreases along with hyponatremia
               in the euvolemic patient .
                                    [14]

               An appropriate intervention depends on determining the timing of hyponatremia onset, the severity of the
               neurological symptoms, and the volemic classification [Table 4]. This information, together with clinical
               history and blood and urine tests, is the basis for determining the etiology of hyponatremia [Table 3].

               Note that a single patient could experience different volemic episodes (e.g., hypovolemic after having
                                         [19]
               been euvolemic) sequentially . That is why clinical examination remains necessary to assure correct
               management of hyponatremia at any given point of time, and reevaluation of patients is essential.

                                                                                                       [20]
               Hyponatremia in oncology patients is often considered primarily euvolemic, secondary to SIADH .
               However, some studies have found that hypovolemic or hypervolemic hyponatremia is more prevalent in
                                       [3,6]
               hospitalized cancer patients .

                                                                                                   [21]
               The clinician should remember that the diagnosis of SIADH is always a diagnosis of exclusion , in a
               euvolemic patient with a urine osmolality higher than 100 mOSm/kg, in the absence of pain, nausea,
               diuretics, adrenocorticotropic hormone (ACTH) deficit, diuretic use, or severe hypothyroidism. ACTH
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