A 35-year-old man with T1DM is dehydrated and has a blood pressure of 90/50 mmHg. What is the most appropriate initial investigation?

In the scenario described, the 35-year-old man with type 1 diabetes mellitus (T1DM) is exhibiting signs of dehydration and hypotension, which raises concern for potential diabetic ketoacidosis (DKA) or hyperglycemic hyperosmolar state (HHS). Initial investigations play a crucial role in the management of these conditions.

An arterial blood gas (ABG) assessment is the most appropriate initial investigation because it provides critical information about the patient's acid-base status, metabolic state, and levels of oxygen and carbon dioxide in the blood. In cases of DKA, the ABG can reveal metabolic acidosis with a decreased pH and bicarbonate levels. Additionally, it can help in determining the patient's respiratory status, especially if they are in distress.

While other tests such as CBC or serum urea might provide useful information about the patient's overall condition and renal function, they do not offer the same immediate insight into the potential metabolic derangements that are common in an acute worsening of diabetes. HbA1c, measuring average blood glucose levels over the past two to three months, is not useful in an acute setting where immediate management decisions need to be made.

Therefore, starting with an ABG is essential for guiding the immediate treatment of this