Chapter 13 Fantus Test
The Fantus test is a simple test for measuring urinary chlorides. It is used to estimate dietary sodium chloride intake. For this purpose, it has been shown to give clinically useful results if two factors are taken into consideration1–3:
• Concentration of urinary chloride is proportional to urinary sodium except with increased ingestion of nonsodium chloride salts (e.g., ammonium chloride or potassium chloride).
• Urinary output of sodium chloride may not be a reliable indicator of sodium intake when an individual has a salt-wasting disorder (Box 13-1).
BOX 13-1 Sodium-Wasting Conditions in Which Salt Restriction Can Be Deleterious
The Fantus test was first described by J.B. Fantus in 1936.4 It was originally used for the diagnosis of salt and water depletion and for the differential diagnosis of edema.5 However, this test has been found to be particularly useful for monitoring sodium intake in individuals with blood pressure disorders, including both hypotension and hypertension.
Since 1904 when Ambard and Beaujard6 first implicated salt as a major factor in the pathogenesis of hypertension, conflicting opinions and data have confused this issue. In 1944, Kempner7 published a classic article describing the effective treatment of essential hypertension using a salt-restricted “Rice Diet.” Many studies have subsequently demonstrated the effectiveness of salt-restricted diets in treating hypertension.8–12 Substantial evidence now exists that sodium alone does not raise blood pressure as had been previously thought, but rather that chloride also plays an important role in the elevation of blood pressure when consumed as sodium chloride, or common table salt.13,14 Sodium bicarbonate has not been found to have the same hypertensive effect as sodium chloride.
The Fantus test is an indirect test for sodium and actually measures chloride ion concentration. Considering that sodium chloride intake, rather than sodium ion alone, is a significant factor in blood volume expansion and blood pressure elevation, the Fantus test may be more clinically useful than direct tests for urinary sodium when managing hypertension or fluid retention.13,14
The clinical utility of limiting salt is obvious; however, when managing hypertension with salt restriction, two difficulties arise.15 First, effective salt restriction is difficult to maintain because of the prevalence of highly salted foods, particularly in convenience and prepared foods.
Secondly, salt must be restricted to quite low levels to be effective in the treatment of hypertension. Sodium chloride, measured in terms of sodium, must be restricted to approximately 3 g/day, compared with the 12 to 27 g of sodium per day commonly consumed in the typical Western diet.
Because the Fantus test is an inexpensive, simple in-office test, it can be effectively used to monitor salt intake where frequent sampling is useful in the management of salt-restricted diets. Salt restriction may be therapeutic in several disorders, including hypertension, congestive heart failure, premenstrual syndrome, edema, and chronic kidney disorders.
The Fantus test is also useful in monitoring salt intake in clinical conditions where restricted sodium chloride intake can be an aggravating factor, such as in Addison’s disease, postural hypotension, chronic fatigue syndrome, and sodium depletion (see Box 13-1). Another use of the Fantus test is in the identification of people who are ingesting excessive amounts of salt. These individuals may be without overt symptoms or signs of disease. It has been observed in animal studies that a high intake of sodium chloride decreases longevity as a factor separate from blood pressure.16 In another study, salt-induced angiotensin elevation was found to adversely affect the myocardium in humans, even when blood pressure remained within normal limits.17 It has been observed that the adverse effects of excess salt can persist for an extended time after excess salt ingestion has been discontinued.18,19
Identifying people at risk for harmful effects from excessive salt consumption is an important aspect of preventive medicine. People with risk factors for hypertension can prudently restrict salt intake. Included in this group of people at risk are individuals with chronic renal disease, those with a family history of hypertension, and individuals over the age of 50.20