Urinalysis, renal function and paediatric kidney assessment — formative SAQs

SAQ 1 — Bedside Schwartz eGFR and staging (10 marks)

A 6-year-old boy is 115 cm tall. His serum creatinine is 0.9 mg per dL, reported on a modern enzymatic assay. He is well, and the test was done as follow-up for an isolated abnormality found at screening. [1] [4]

Questions

1. State the bedside Schwartz equation in full with units, calculate this child's estimated GFR, and state the KDIGO category. (5 marks) [1] [4]

2. Explain why a measured height is essential, and what happens if the original 0.55 constant is used instead of 0.413. (3 marks) [1]

3. State one reason the estimate might be inaccurate despite correct arithmetic. (2 marks) [1]

Model answer

Calculation (5). The bedside Schwartz equation gives eGFR in mL per min per 1.73 m2 equal to 0.413 times height in cm divided by serum creatinine in mg per dL. That is 0.413 times 115 divided by 0.9, equal to 0.413 times 127.8, about 53 mL per min per 1.73 m2. A stable value of 53 falls in KDIGO category G3a, which covers 45 to 59 mL per min per 1.73 m2, and warrants nephrology referral. [1] [4]

Height and constant (3). Height replaces muscle mass in the equation, so a guessed height produces a guessed GFR. The original 0.55 constant, or 0.45 for infants, was derived with older creatinine assays that read higher than current enzymatic methods; using 0.55 today would overestimate the GFR and falsely reassure. [1]

Inaccuracy (2). Reduced muscle mass, such as in neuromuscular disease or cachexia, lowers creatinine generation and drifts the estimate upward; cystatin C would then be a better cross-check. [1]

SAQ 2 — Urine collection, dipstick and the creatinine lag (10 marks)

A febrile 18-month-old is not yet toilet-trained. A bag-urine culture grows a mixed growth of coliforms. Separately, a dehydrated, oliguric 4-year-old has a serum creatinine within the reference range. [3] [5]

Questions

1. Explain why the bag-urine result cannot be used to confirm urinary tract infection, and state the preferred collection method. (4 marks) [3] [6]

2. Describe how to read the dipstick and microscopy together to localise the source of a positive blood result. (3 marks) [2]

3. Explain why a normal serum creatinine does not exclude acute kidney injury in the oliguric child, and state the safer approach. (3 marks) [5]

Model answer

Bag urine (4). Bag urine carries a high contamination rate, so a mixed growth of coliforms more often reflects perineal skin flora than true infection. For diagnosis where the result changes treatment, use clean-catch, catheterisation or suprapubic aspiration, which cut contamination. Comparison of catheter and clean-catch samples in preschool children shows acceptable and similar results when the technique is good. [3] [6]

Dipstick and microscopy (3). The blood pad is positive for intact red cells, free haemoglobin or myoglobin, so the strip alone cannot localise the source. Microscopy separates them: dysmorphic red cells and red cell casts point to the glomerulus, while normal red cells point to the lower tract. The dipstick raises the hypothesis and the sediment confirms it. [2]

Creatinine lag (3). Creatinine rises only after filtration has already fallen and muscle has had time to load the blood, so a single normal value under-represents early injury. The safer approach is to treat the urine output and trend the creatinine over hours to days, while managing the cause and the fluid and electrolyte state. [5]