What is Hypouricemia?
Diagnosis of Hypouricemia
Hypouricemia has no symptoms other than low uric acid levels. There are 2 types of hypouricemia: renal hypouricemia in which uric acid generated in the body is excessively excreted, and xanthinuria in which enough uric acid cannot be generated due to deficiency of the enzyme that degrades purines into uric acid. In Japan, the majority of hypouricemia is comprised of renal hypouricemia. Furthermore, secondary hypouricemia may be developed as a result of other diseases.
Diseases Causing Hypouricemia
There are 2 types of hypouricemia: hypouricemia with hyperactive excretion of uric acid (where uric acid levels become lower as the uric acid generated in the body is rapidly excreted) and hypouricemia with reduced uricopoiesis (where uric acid levels become lower due to insufficient generation of uric acid).
Hypouricemia with Hyperactive Excretion of Uric Acid (Uric acid is rapidly excreted)
- Renal Hypouricemia
- Fanconi syndrome
- Wilson’s disease
- Syndrome of inappropriate antidiuretic hormone secretion
- Malignant Tumor
- Drug (e.g.: Benzbromarone, Probenecid)
- Intractable Diarrhea
Hypouricemia with Reduced Uricopoiesis (Uric acid is not generated in the body)
- Molybdenum Cofactor Deficiency
- PRPP Synthetase Hypoactivity
- Idiopathic Hypouricemia with Reduced Uricopoiesis
- Severe Renal Failure
- Drugs (e.g.: Allopurinol)
- Emaciation (Undernutrition)
Among the above, renal hypouricemia and xanthinuria are not secondary hypouricemia induced by drugs or other factors and have no symptoms which require treatments other than the presence of lower uric acid level. In Japan, renal hypouricemia is the dominant condition.
Diagnosis of Renal Hypouricemia
To diagnose renal hypouricemia, blood uric acid level measurement is conducted multiple times to confirm that the uric acid level is lower than 2.0 mg/dL and then the excretion efficiency of blood uric acid into urine (uric acid clearance) and the excretion rate of uric acid into urine (uric acid excretion rate) must be observed.
In the human body, uric acid is resorbed through the renal tubule, and approximately 70% of uric acid that is excreted outside of the body is excreted from the kidneys into the urine. However, in the case of patients with renal hypouricemia, uric acid is not sufficiently resorbed due to a mutation of the gene controlling the resorption of uric acid through the renal tubule and is excreted with urine. Therefore, uric acid excretion efficiency (uric acid clearance and uric acid excretion rate) is characteristically increased.
However, since uric acid levels can be temporarily decreased by various causes, multiple measurements are required.
Diagnosis of Xanthinuria
With lower prevalence, there are patients with xanthinuria instead of renal hypouricemia. Uric acid levels in patients with xanthinuria are lower due to the deficiency of enzymes that degrade purines into uric acid.
Since xanthinuria presents very low levels of uric acid in urine, the distinction between renal hypouricemia and xanthinuria is possible with urinalysis.