The Validity of Fractional Excretion of Uric Acid in the Diagnosis of Acute Kidney Injury Due to Decreased Kidney Perfusion

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To the Editor:

The validity of the fractional excretion of sodium (FE_Na) for the diagnosis of acute kidney injury (AKI) caused by decreased kidney perfusion is limited in patients receiving diuretics. Fractional excretion of uric acid (FE_UΑ) may be useful in such cases.¹, ², ³

We studied the validity of FE_UΑ in 44 patients, 22 (15 men and 7 women; mean age, 57.4 ± 16.2 [SD] years) with AKI caused by decreased kidney perfusion and 22 (14 men and 9 women; mean age, 54.2 ± 14.5 years) with AKI from other causes.

We measured sodium (Na), creatinine (Cr), and uric acid (UA) in serum (S) and a spot urine sample (U) and calculated FE_Na and FE_UA using the formulas (Na_U · Cr_S/Cr_U · Na_S) · 100 and (UA_U · Cr_S/Cr_U · UA_S) · 100, respectively. No patient had used diuretics. The diagnosis of AKI from decreased kidney perfusion was based on: (1) clinical manifestations of hypovolemia, oliguria, or significant decrease in urine output; (2) absence of urine findings indicating intrinsic renal injury; and (3) quick restitution of urine output, as well as renal function, after correction of the hemodynamic disturbances.

FE_Na and FE_UΑ were significantly lower in the patients with AKI caused by decreased kidney perfusion. For these patients, the limit of detection for FE_Na was 2.47% (with 91% sensitivity and specificity), and for FE_UA, 23.79% (82% sensitivity/specificity). In the comparison of receiver operating characteristic curves, there was a trend (P = 0.085) toward greater accuracy of FE_Na than FE_UA (Fig 1). In conclusion, calculation of FE_UA may be a useful method for the diagnosis of AKI caused by decreased kidney perfusion.

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• Figure 1. 

  Area under the receiver operating characteristic (ROC) curve for fractional excretion of sodium (FE_Na; upper plot) and uric acid (FE_UA; lower plot). P = 0.09.

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Acknowledgements 

The corresponding author, Dr Kosmadakis, may be contacted at george.kosmadakis@googlemail.com.

Support: None.

Financial Disclosure: None.

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References 

1. Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984;77(4):699–702
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2. Quarantino CP, Di Sciascio N, Rucci C, Ciaglia P, Giacomello A. The normal range of serum urate levels and of fractional urate excretion. Adv Exp Med Biol. 1994;370:91–93
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3. Cappuccio FP, Strazzyllo P, Farinaro E, Trevisan M. Uric acid metabolism and tubular sodium handling (Results from a population-based study). JAMA. 1993;270(3):354–359
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