Urinary pH, Renal Failure & Dosing the Real Patient

Urinary pH and ion trapping

Recall from guide 2 that only un-ionized drug is reabsorbed back into the blood; the ionized form is stuck in the tubule and washed out. The pH of the urine controls how much of a drug is ionized — this is the urinary pH effect. When a drug becomes ionized in the urine and can no longer diffuse back, we say it is trapped: ion trapping.

Rule: a drug is ionized (trapped) when urine pH is on the OPPOSITE side
      of the drug's nature.

  Weak ACID  (e.g. aspirin, phenobarbital)
     -> ionized in ALKALINE urine  -> trapped  -> excreted faster
     -> treat overdose by ALKALINIZING urine (sodium bicarbonate)

  Weak BASE  (e.g. amphetamine)
     -> ionized in ACIDIC urine     -> trapped  -> excreted faster
     -> (acidifying urine is rarely used clinically now)

Mnemonic: "acid in base, base in acid" gets trapped & cleared.

Measuring kidney function

To dose renally cleared drugs safely, you need to know how well the kidney is working. The bedside proxy is creatinine clearance (CrCl). Creatinine is a muscle waste product that is filtered freely and barely reabsorbed, so its clearance approximates GFR. The Cockcroft–Gault estimate lets you get CrCl from age, weight, sex, and a blood creatinine — no urine collection needed.

Cockcroft-Gault (estimates CrCl in mL/min):

  CrCl = (140 - age) x weight(kg) / (72 x serum creatinine[mg/dL])
         ... x 0.85 if female

Example: 70 yr, 60 kg, female, Scr = 1.2 mg/dL
  = (140-70) x 60 / (72 x 1.2) x 0.85
  = 4200 / 86.4 x 0.85
  ~= 48.6 x 0.85  ~=  41 mL/min   (reduced -> adjust renal drugs)

Adjusting the dose when the kidney fails

When the kidney is failing, a renally cleared drug is removed more slowly — clearance drops, half-life lengthens, and the drug accumulates to dangerous levels at the old dose. This is the core of renal impairment dosing. You have two levers, and you can use either or both.

1. Reduce the dose (give less each time) — keeps peaks from getting too high. Good when you need steady levels.
2. Lengthen the interval (dose less often) — lets the slow kidney catch up between doses. Good for drugs that need high peaks but low troughs (e.g. aminoglycosides).
3. Note the loading dose usually stays the same — it fills the volume of distribution, which renal failure doesn't change; only the maintenance dose is cut.
4. For narrow-window drugs, confirm with therapeutic drug monitoring — measure blood levels and adjust to the real patient.

Tie it back to guide 4: failing kidneys lower renal clearance, so total clearance falls, half-life rises, and steady state takes longer to reach and sits higher unless you cut the dose. The whole track converges here — knowing the exit route, the renal mechanisms, and the three numbers lets you keep a real patient both effective and safe.