Proximal Renal Tubulopathy (PRT)

Indications and Tests for Proximal Renal Tubulopathy (PRT)

Indications for proximal renal tubulopathy tests	Proximal renal tubulopathy tests^(iv), including	Replace TDF by non-tenofovir drug or TAF* alternative drug if:
Progressive decline in eGFR⁽ⁱ⁾& eGFR ≤ 90 mL/min & no other cause and/or Confirmed hypophosphataemia⁽ⁱⁱ⁾and/or Confirmed increase in UP/C⁽ⁱⁱⁱ⁾ Renal insufficiency even if stable (eGFR ≤ 60 mL/min) Tubular proteinuria^(v)	Blood phosphate and urinary phosphate excretion^(vi) Blood glucose and glycosuria Serum bicarbonate and urinary pH^(vii) Blood uric acid level and urinary uric acid excretion^(viii) Serum potassium and urinary potassium excretion	Confirmed proximal renal tubulopathy with no other cause

For eGFR: use CKD-EPI formula. The abbreviated MDRD (Modification of Diet in Renal Disease) or the Cockcroft-Gault (CG) equation may be used as an alternative, https://www.chip.dk/Tools-Standards/Clinical-risk-scores
Serum phosphate < 0.8 mmol/L or according to local thresholds; consider renal bone disease, particularly if alkaline phosphatase increased from baseline: measure 25(OH) vitamin D, PTH
UP/C in spot urine, detects total urinary protein, including protein of glomerular or tubular origin. The urine dipstick analysis primarily detects albuminuria as a marker of glomerular disease and is inadequate to detect tubular disease
It is uncertain which tests discriminate best for TDF renal toxicity
Proximal tubulopathy is characterised by: proteinuria, hypophosphataemia, hypokalaemia, hypouricaemia, renal acidosis, glucosuria with normal blood glucose level. Renal insufficiency and polyuria may be associated. Most often, only some of these abnormalities are observed
Tests for tubular proteinuria include retinol binding protein, α1- or β2-microglobulinuria, urine cystatin C, aminoaciduria.
Quantified as fractional excretion of phosphate (FEPhos): (PO4(urine) / PO4(serum) / (Creatinine(urine) / Creatinine(serum) in a spot urine sample collected in the morning in fasting state. Abnormal > 0.2 (> 0.1 with serum phosphate < 0.8 mmol/L)
S-bicarbonate < 21 mmol/L and urinary pH > 5.5 suggests renal tubular acidosis
Fractional excretion of uric acid (FEUricAcid): (UricAcid(urine) / UricAcid(serum) / (Creatinine(urine) / Creatinine(serum) in a spot urine sample collected in the morning in fasting state; abnormal > 0.1

* There are limited data on use of TAF with eGFR ≤ 10 mL/min