Laboratory Tests in the Diagnosis of Metabolic Acidosis
| QUESTION | PARAMETER ASSESSED | TOOLS TO USE |
|---|---|---|
| Is the content of HCO 3 − low in the ECF? | ECF volume | Hematocrit or total plasma proteins |
| Have new acids accumulated? | Appearance of new anions in the body or the urine | P A nion gap Urine anion gap |
| Are toxic alcohols present? | Detect alcohols as unmeasured osmoles | P Osmolal gap |
| Is the renal response to acidemia adequate? | Examine the rate of excretion of NH 4 + | Urine osmolal gap |
| If NH 4 + excretion is high, which anion was excreted? | GI loss of NaHCO 3 Acid added, but its new anions are excreted | Urine Cl − is high Urine anion gap |
| What is the basis for a low rate or excretion of NH 4 + ? | Low net distal H + secretion Low NH 3 availability Both defects | Urine pH >6.5 Urine pH ∼5.0 Urine pH ∼6.0 |
| Where is the defect in H + secretion? | Distal H + secretion Proximal H + secretion | PCO 2 in alkaline urine FE HCO3 , U Citrate |
ECF , Extracellular fluid; GI , gastrointestinal.
