Urate

Diagnostic Use

Uric acid is increased in gout, renal failure, polycythaemia, and a number of neoplastic disorders where there is a significantly increased cell turnover (e.g. leukaemia, myeloma, lymphoma).

Diagnosis of Gout:

1. The 5 year cumulative incidence of asymptomatic hyperuricaemia developing into first gout attack is around 2% if the serum urate is at 0.42 mmol/L, rises to 20% if serum urate is above 0.53mmol/L

2. The risk of recurrent gout attack and recurrent tophi deposition is lower if serum urate level is repeatedly below 0.36 mmol/L.

3. Serum urate levels often fall during the acute attack so a normal serum urate does NOT exclude the diagnosis of gout at this time.

4. A raised serum urate level is not sufficient to make the diagnosis of gout, which requires:

(a) classical history of podagra
(b) identification of urate crystals in joint fluid, or
(c) (in late cases), typical X-ray changes

Hypouricaemic Therapy
The aim of hypouricaemic therapy, irrespective of gender, for long term prevention of acute gout attack and for tophi regression is to consistently hold the serum urate level  below 0.36 mmol/L.  If renal impairment is present, careful titration of allopurinol aim to achieve urate below 0.36mmol/L while at the same time maintaining close surveillance on renal function is advisable –  consider seeking advice from rheumatologist and/or renal physician.

Other than for gout-related purposes, serum urate is also utilised in other clinical settings:

a. Assessment of pre-eclampsia

b. Assessment of metabolic syndrome

c. Evaluation of syndrome of inappropriate anti-diuretic hormone (including those on diuretics)

d. Monitor tumour lysis syndrome

e. Investigation of inborn errors of metabolism

f. Assessment of risk of renal stone formation

Significant renal disease in form of chronic interstitial nephritis and progressive renal failure independently due to chronic hyperuricaemia if at all, is likely rare. There is still considerable debate and interest that may be in some individuals, chronic urate nephropathy is possible. It can be suspected if there are features of chronic renal insufficiency, bland urine sediment, hyperuricaemia disproportion to degree of renal insufficiency (plasma urate >0.59mmol/L when creatinine is between 132-176umol/L; plasma urate >0.71mmol/L when creatinine is >176umol/L) and after lead poisoning has been excluded.  In a specific subgroup of chronic urate nephropathy, the Familial Juvenile Hyperuricaemia nephropathy (autosomal dominant), there are features of family history of renal failure, early occurrence of gout/hyperuricaemia disproportion to degree of renal insufficiency, early and progressive renal insufficiency, bland urinalysis and hypertension. In another rare subgroup, the X linked Lesch-Nyhan syndrome with absent HGPRT, hyperuricaemia and hyperuricosuria, there is high incidence of chronic kidney disease.

Reference Intervals

Units: mmol/L

Reference range:

0 – 1 years                      0.08 – 0.37

1 – 5 years                      0.10 – 0.30

Males :

5 – 10 years                    0.13 – 0.33

>10                                   0.23 – 0.42

Females :

5 – 13 years                     0.13 – 0.33

13                                   0.16 – 0.42

• Changes even within the reference interval sometimes can already be significant in preclampsia of pregnancy

Conversion factors:  mg/100 mL x 0.0598 = mmol/L
mmol/L x 16.722 = mg/100 mL