example calculation winters formula agma

A chemistry panel showed the following levels: sodium, 140 mEq/L (140 mmol/L); potassium, 4.0 mEq/L (4.0 mmol/L); chloride, 105 mEq (105 mmol/L); bicarbonate, 12 mEq/L (12 mmol/L); glucose, 122 mg/dL (6.77 mmol/L); BUN, 18 mg/dL; creatinine, 1.4 mg/dL (124 μmol/L); and albumin, 4.2 g/dL (42 g/L).

Trace levels of ketones are noted in her urine. Serum osmolality is 294 mOsm/kg (294 mmol/kg). Lactate level is 1.44 mg/dL (1.6 mmol/L).

Arterial blood gas analysis revealed a pH of 7.27, Pco2 of 20 mm Hg, and Po2 (room air) of 99 mm Hg.

Salicylates are found in hundreds of over-the-counter medications and in numerous prescription drugs, making salicylate toxicity an important cause of morbidity and mortality. This is especially true in the older patient population in which, according to some studies, systemic clearance of salicylate (mainly by hepatic metabolism) is reduced with age, as is renal elimination. In the face of renal impairment, the risk of toxicity is increased. Chronic toxicity, which can occur even with marginally high salicylate concentrations, can vary between mild transient symptoms to a more severe "pseudosepsis" syndrome with fever, leukocytosis with increased band forms, as well as hypotension, with a reduced systemic vascular resistance. These cases are treated with drug withdrawal and supportive therapy. Chronic salicylate toxicity can be averted by adjusting the dose of medications with salicylates, avoiding ingestion of multiple different salicylate preparations concurrently, and monitoring clinical and laboratory parameters to guide dosage. Renal function should be monitored to detect nephrotoxicity from chronic salicylate therapy. In addition, patients should be regularly screened for evidence of gastrointestinal bleeding.

In this case, the patient has a high anion gap metabolic acidosis and a coexisting respiratory alkalosis. This combination, combined with the fact that the patient has noted tinnitus, suggests salicylate toxicity (choice C is correct).

We know the primary disturbance is an acidosis by looking at the arterial blood gas analysis and noting the pH of 7.27. When addressing the laboratory data, first calculate the anion gap as follows:

Anion gap = [Na+] – [Cl−] – [HCO3−]

In this case, the anion gap would be 140 − 105 − 12 = 23. A normal anion gap is 11, making this a high anion gap metabolic acidosis. The difference between what the anion gap is and what is expected to be is 23 − 11 = 12 (otherwise known as the Δ anion gap). If the anion gap is too high by 12, we would expect the serum bicarbonate to fall by 12. Because a normal bicarbonate level is 24 mEq/L (24 mmol/L), we would expect the bicarbonate level to drop to 12 mEq/L (12 mmol/L) (24 − 12 = 12), which is the precise level measured in this case.

If the arterial blood gas is now reviewed, using Winter's formula to calculate the "expected" Pco2, 

Pco2 = (1.5 x HCO3−) + 8 ± 2

we would expect the Pco2 to drop in proportion to the drop in bicarbonate level. The Pco2 is 20 mm Hg and a normal Pco2 is 40 mm Hg; therefore, the Pco2 in this case is too low by 20 mm Hg. The bicarbonate level in this case only dropped by 12 mEq/L (12 mmol/L; meaning the Pco2 should have been 28 mm Hg ± 2, or 40 − 12 = 28). Therefore, there is a respiratory alkalosis in addition to the high anion gap metabolic acidosis.

An osmolar gap is sometimes calculated if there is a clinical suspicion of unmeasured osmoles in the circulating blood, seen with ingestions of toxins such as methanol or ethylene glycol. The osmolar gap is the measured osmolality minus the calculated osmolality. In this case, the calculated osmolar gap would be 2 [Na+] + BUN/2.8 + glucose/18, or 280 + 6 + 7 = 293. The measured osmolality was 294, suggesting no osmolar gap was present. This makes ethylene glycol toxicity unlikely (choice D is incorrect).

Metformin is in the biguanide class of medications involving an oral medication to lower blood sugar levels. A rare adverse effect of metformin is development of lactic acidosis. In this case, the creatinine level essentially is normal, as is the lactate level (choice B is incorrect). In addition, the glucose and serum electrolyte levels are essentially normal and not indicative of poorly controlled hyperglycemia; therefore, diabetic ketoacidosis is unlikely as well (choice A is incorrect).1

Footnotes

  1. SEEK Questionnaires ↩