Renal function and blood chemistry
creatinine clearanceeGFRanion gapcorrected calciumrenal dose adjustment
Creatinine clearance, eGFR, anion gap, corrected calcium and dose rate from clearance: reading a panel and adjusting to it.
Creatinine Clearance (Cockcroft-Gault)
Cockcroft and Gault's 1976 estimate of creatinine clearance in mL/min from age, body mass, serum creatinine and a sex factor.
Estimated GFR (MDRD 4-Variable, IDMS-Traceable)
Estimated glomerular filtration rate in mL/min per 1.73 m² by the IDMS-traceable four-variable MDRD equation.
Serum Anion Gap
The difference between the measured cation and the measured anions in serum, the standard first split in the workup of a metabolic acidosis.
Corrected Calcium for Albumin
Total serum calcium adjusted upward for low albumin, since roughly 40 % of calcium travels bound to albumin and is not biologically active.
Maintenance Dose Rate from Clearance
The rate of drug administration that exactly replaces what clearance removes, holding a steady-state plasma concentration.
How they fit together
The two kidney equations answer slightly different questions and are not interchangeable, which is the single most useful thing to take from this set. Cockcroft-Gault estimates creatinine clearance in mL per minute for the actual person, weight included, and it is the one the great majority of drug monographs were written against. MDRD estimates glomerular filtration rate normalised to 1.73 square metres of body surface, which is right for staging chronic kidney disease and wrong for dosing anyone far from average size. Use Cockcroft-Gault when a dose depends on the answer, and check what the monograph specifies rather than assuming.
Both rest on serum creatinine, which is produced by muscle. That is the assumption that fails quietly. A frail elderly patient with little muscle mass can carry a normal-looking creatinine on badly reduced kidney function, and a heavily muscled young adult can look impaired while being fine. Neither equation is valid at all when creatinine is still moving, which is exactly the case in acute injury. Maintenance dose rate is here because it is what the clearance figure is usually for, and it is the direct link from a lab value to a prescription.
The other two correct measurements that mislead as reported. Corrected calcium exists because roughly 40% of serum calcium travels bound to albumin and is not biologically active, so a low albumin makes total calcium read low while the ionised fraction that matters is untouched. Add 0.8 mg/dL for every 1 g/dL of albumin below 4. The anion gap is the first split in any metabolic acidosis workup, separating causes that add an acid from those that lose bicarbonate, and it needs the same albumin correction for the same reason: albumin is itself an unmeasured anion, so a low albumin hides a raised gap.