Exam Questions Flashcards
The clinical chemistry laboratory plays an important role in the diagnosis and monitoring of renal disorders. Discuss
the tests that are part of a renal profile.
A multitude of tests comprise the renal profile. An especially prevalent test is that of creatinine, both in serum and urine. Creatinine has a reference range of 80 - 145 µmol/L for males, and 62 - 97 µmol/L for females. Urine creatinine may be measured by both enzymatic and chemical methods (although enzymatic methods measure creatininase, not creatinine). Jaffe’s technique, a chemical method, involves the reaction of urine with alkaline picrate, yielding an orange-red complex, which can then be read spectrophotometrically, to find the creatinine concentration. Sources of interference in the Jaffe method include protein, glucose, and bilirubin, and care must be taken to accommodate this. As creatinine is normally freely filtered by the glomeruli of the kidneys, an increase can indicate renal disease or injury. Serum and urine creatinine concentrations are required to calculate the glomerular filtration rate (GFR), which also uses a 24-urine collection volume, in the formula GFR in mL/min = (urine creatinine)(urine (mL/min)) / serum creatinine. GFR is a strong indicator of any issues with renal function, as it can highlight diminished urinary output. Estimated GFR (eGFR) negates the requisite 24-hour urine collection, but requires more extensive calculation. Urea, another analyte, is regularly measured in tandem with creatinine; the reference range for blood urea is 2.8 - 6.8 mmol/L. Urea is most commonly measured enzymatically, and the Berthelot method uses urease to convert urea into ammonia and water; ammonia is then reacted with phenol and hypochlorite in alkaline media, to form coloured indophenol. The complex is read spectrophotometrically. Urea concentration can also be determined using glutamate dehydrogenase, which converts ammonia to glutamate and water, and the reaction is coupled to oxidation of NADH, giving rise to the formation of a coloured complex, which may then be monitored. Proteinuria (the presence of protein in the urine), can be detected by colourimetric, dye-binding dipsticks, and the Biuret method, which sees cupric ions react with peptide bonds to form a violet complex. Osmolality, a measure of the concentration of urine, can be quantified by an osmometer, and if the urine:plasma osmolality ratio is not 1:1, this indicates that water is not being reabsorbed, and therefore suggestive of renal impairment. Urine osmolality depends on antidiuretic hormone, and an osmolality of ≥600mOsm/kg is normal. Urinary pH is used to diagnose renal tubular acidosis, while specific proteinuria measures β2 microglobulin and α1 microglobulin. Finally, laboratories generally include measures of electrolytes such as potassium, chloride, and sodium (quantified using the iron selective electrode method) in the creation of a renal profile.
