Inorganic sulfate is involved in metabolic processes of activation and detoxification of endogenous and exogenous compounds Its plasma levels are maintained fairly constant (0.2-0.6 mM/L).
Sulfate is excreted mainly by the kidney, via free glomerular filtration, followed by proximal tubular reabsorption with a saturable mechanism (Figure 1).
Sulfate retention as a consequence of chronic kidney disease has long been known. However, no studies have been done in adults to evaluate the relationship with GFR.
To evaluate the relationship between plasma inorganic sulfate concentrations and the level of GFR impairment in chronic kidney disease (CKD) patients.
Patients and Methods
Sixty-four adult CKD patients (30 males, age 25-85 years, serum creatinine 0.51-6.65 mg/dL).
GFR was measured in fasting patients as the renal clearance of 99mTc-DTPA, at the same time a blood sample was drawn for biochemical measurements.
Plasma creatinine was measured with a standard laboratory method.
Plasma sulfate levels were determined with a turbidimetric method (Sulfate Assay Kit, Sigma Aldrich®).
Plasma sulfate ranged 0.5-2.32 mM/L (mean 0.94) in men and 0.33-2.03 mM/L (mean 0.84) in women.
With decreasing GFR sulfate rose exponentially to a maximum of 2.32 mM/L (GFR 6.3 mL/min 1.73 m2). The large majority of patients with GFR < 40 mL/min 1.73 m2 had a plasma sulfate level > 1 mM/L.
A significant correlation was found between plasma sulfate and creatinine (r=0.674). A significant correlation (r=0.651) was also found with GFR. The correlation was closer among men (r=0.70) then women (r=0.58). This finding could be due to a different tubular handling of sulfate related to sex: due to a larger number of tubular transporters, women have higher tubular reabsorption of sulfate which is saturated at higher plasma levels.
The correlation of serum creatinine with GFR (r=0.874) was significantly closer than that of plasma sulfate with GFR (r=0.651) (Figure 2).
Plasma sulfate correlates with GFR.
Other factors affect this relationship: saturable tubular reabsorption and possibly tubular secretion of sulfate, together with the amount of dietary sulfate.
In any case, plasma levels of sulfate should be taken into account in CKD patients, due to their relevance in acid/base equilibrium and detoxication mechanisms.