Differential Diagnosis

The patient's presentation includes a high blood glucose level (721), slightly alkalotic pH (7.41), low bicarbonate level (20), no anion gap, elevated lactic acid (4.2), and a creatinine level of 1.5, with a history of forgetting to take Lantus (a long-acting insulin) the previous night. Based on these findings, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis
  • Diabetic Ketoacidosis (DKA): Although the pH is slightly alkalotic and there's no anion gap mentioned, the high blood glucose, elevated lactic acid, and the context of missing insulin (Lantus) suggest DKA as a primary consideration. However, the absence of an anion gap and the alkalotic pH are atypical. It's possible that the patient is in a mixed state or that the DKA is not fully developed, given the recent omission of insulin.
• Other Likely Diagnoses
  • Hyperglycemic Hyperosmolar Nonketotic Syndrome (HHNS): Given the very high glucose level and the absence of significant ketosis (implied by no anion gap), HHNS could be considered, especially in the context of dehydration and elevated creatinine.
  • Lactic Acidosis: The elevated lactic acid level could suggest lactic acidosis, which can occur in the context of diabetes, especially if there's tissue hypoperfusion or sepsis. However, the lack of anion gap and the alkalotic pH make this less straightforward.
• Do Not Miss Diagnoses
  • Sepsis: Although not directly indicated by the lab values provided, sepsis can cause lactic acidosis, hyperglycemia, and acute kidney injury (suggested by the elevated creatinine). It's crucial to consider and rule out sepsis due to its high mortality if untreated.
  • Acute Kidney Injury: The elevated creatinine suggests renal impairment, which could be due to dehydration, sepsis, or other causes. It's essential to address the renal function promptly.
• Rare Diagnoses
  • Other Causes of Lactic Acidosis: Such as mitochondrial disorders or certain medications, though these would be less likely given the context of diabetes and missed insulin dose.
  • Mixed Acid-Base Disorders: The patient's presentation could suggest a mixed disorder, given the alkalotic pH and low bicarbonate. However, further evaluation would be needed to determine the exact nature of any acid-base disturbance.

Each of these diagnoses requires careful consideration of the patient's clinical context, including the history of diabetes, the omission of insulin, and the presence of elevated lactic acid and creatinine. A thorough evaluation, including additional laboratory tests and clinical assessment, is necessary to determine the most accurate diagnosis and guide appropriate treatment.