investigation

Last reviewed 09/2021

Investigation in Primary Care.

The possibility of a urinary tract infection or diabetes mellitus should be excluded.

• quantification of proteinuria
  • calculate a spot urine protein:creatinine ratio from a single (preferably early morning) urine specimen (this is simpler than a 24 hour urine collection, and almost as accurate)
    • 24 hour urinary protein excretion (mg per 24 hours) can be approximated as (mg/l protein) ÷ (mmol/l creatinine)x10
      • if the estimated protein excretion is more than 150 mg in 24 hours (equivalent to a protein:creatinine ratio of 15 mg/mmol) then this result is abnormal
        • if the degree of proteinuria is in the nephrotic range (> 3.5 g/24 h or a ratio > 350) then check serum albumin and cholesterol concentrations.
• assessment of renal function
  • blood tests for renal biochemisty (serum electrolytes, urea, and creatinine)
    • creatinine clearance gives a more accurate representation of renal function than creatinine alone. An estimation of creatinine clearance can be calculated from the Cockcroft-Gault formula:
      • creatinine clearance (ml/min) = ((140 - age)x weight (kg) xC) ÷ serum creatinine (µmol/l), where C = 1.23 in men or 1.04 in women
      • if the estimated creatinine clearance of > 90 ml/minute then this can be considered normal
      • note that creatinine clearance declines with age so lower values may be normal in elderly people and in people with low muscle mass (1)
• in general referral to a nephrologist is indicated if significant proteinuria (proteinuria > 100 mg/mmol)
  • however proteinuria > 50 mg/mmol may be significant if other features of renal disease are present (e.g. impairment of renal function, coexistent microscopic haematuria, hypertension, features indicating an underlying systemic disease) (1)
    • other guidance suggests referral for protein:creatinine ratio >100 mg/mmol, or >45 mg/mmol if co-existing microscopic haematuria or estimated GFR <60mL/min (2)
      • prot/creat ratio at levels <= 45mg/mmol then manage as Chronic Kidney Disease (CKD), according to stage
  • if referring a patient for nephrology review then consider initiating other investigations such as renal tract ultrasonography, immunology (serum and urine protein electrophoresis, antinuclear antibodies, antineutrophil cytoplasmic antibodies, complements), and hepatitis B and C serology

Reference:

1. Haynes J, Haynes R. Proteinuria. BMJ 2006; 332:284.
2. http://www.renal.org/