DKA vs HHS: The Comparison, and the Potassium Rule That Decides the Answer

Both are hyperglycaemic emergencies and both are treated with fluids, insulin and potassium. The order of those three is what the exam scores, and insulin is not first.

Students meet diabetic ketoacidosis and hyperosmolar hyperglycaemic state as a pair of definitions to memorise, then lose marks on something neither definition mentions.

The question is almost never "which one is this". It is what you do first, and the answer is fluids. Insulin comes second, and there is a potassium value below which you do not give insulin at all.

Here is the comparison, then the treatment order, then the one rule that decides more items than everything else combined.

Side by side

The mechanism explains every row. In DKA there is essentially no insulin, so the body burns fat, producing ketones, which are acids. That acid drives the Kussmaul breathing, which is the lungs blowing off carbon dioxide to compensate, and gives the fruity breath.

In HHS there is enough insulin to prevent ketosis but not enough to control glucose. No ketones means no acidosis, which means no Kussmaul respirations and no fruity breath. Instead glucose climbs far higher over a much longer period, pulling enormous volumes of water into the urine. That is why HHS patients are drier, more confused and more likely to die.

The shorthand: DKA is an acid problem, HHS is a water problem.

Treatment, in order

Both conditions follow the same three steps in the same sequence.

1. Fluids first

Isotonic saline, 0.9%, and a lot of it. These patients are profoundly volume depleted, HHS even more so than DKA.

Fluids alone lower glucose substantially by dilution and by restoring kidney perfusion so glucose can be excreted. They also restore blood pressure and tissue perfusion before insulin starts shifting electrolytes around.

Fluids before insulin is the single most tested sequencing point in this topic. An option offering an insulin bolus as the first action for a hypotensive, severely dehydrated patient is wrong, however high the glucose is.

2. Insulin, and only regular insulin IV

Continuous intravenous infusion of regular insulin. It is the only type given intravenously. Options offering subcutaneous insulin aspart or glargine for an acute emergency are distractors.

When blood glucose reaches roughly 200 to 250 in DKA, dextrose is added to the fluids. This surprises students, who assume you stop the insulin. You do not: the insulin is still needed to shut off ketone production and close the anion gap, and the dextrose simply keeps the patient from going hypoglycaemic while it finishes the job.

In DKA, the endpoint is the closed anion gap, not a normal glucose.

3. Potassium

Which brings us to the rule.

The potassium rule

A patient in DKA has lost enormous amounts of potassium in the urine. Total body potassium is depleted. But the serum potassium often reads normal or even high on arrival, because acidosis drives potassium out of cells into the blood, and the lack of insulin keeps it there.

So the number in front of you understates the deficit, badly.

The moment you give insulin, potassium moves back into the cells. Serum potassium can fall off a cliff, and severe hypokalaemia causes fatal arrhythmias.

Hence:

• Potassium below 3.3: hold the insulin. Replace potassium first. Giving insulin here can kill the patient.
• Potassium 3.3 to 5.2: start insulin and add potassium to the fluids.
• Potassium above 5.2: start insulin, hold potassium, recheck frequently.

If you remember one thing from this post, make it that first line. "The glucose is 600 and the potassium is 3.0, what does the nurse do first" is a question with exactly one safe answer, and it is not insulin.

The other dangers

Cerebral oedema if glucose is lowered too quickly. Water follows the osmotic gradient into brain cells. The risk is highest in children and is why controlled correction matters more than fast correction. New headache, falling consciousness or vomiting during treatment is cerebral oedema until proven otherwise.

Hypoglycaemia from continuing insulin without adding dextrose once glucose approaches 200 to 250.

Hypokalaemia, covered above, and the most common serious complication of treatment.

Fluid overload in older patients with cardiac or renal disease, who still need resuscitation but need watching while they get it.

What sets them off

The precipitants are worth knowing because stems lean on them.

For DKA: infection, missed or insufficient insulin, a newly diagnosed type 1, a failed insulin pump, physiological stress such as surgery or myocardial infarction.

For HHS: infection, especially pneumonia and urinary tract infection; a patient who cannot access water independently, which is why it clusters in older adults and residential care; medications including steroids and some diuretics.

Infection is the most common trigger for both. A patient admitted with a urinary tract infection whose glucose then climbs is a stem being set up.

Working one of these items

1. Check ketones and pH. Present and acidotic is DKA. Absent with a glucose above 600 is HHS.
2. Check whether the question asks what to do first. If so, the answer is fluids unless the stem has already given them.
3. Scan for the potassium. Below 3.3 overrides everything: replace before insulin.
4. If insulin is in the answer, confirm it is regular insulin, intravenously.
5. If the glucose is already falling toward 250 in DKA, look for an option adding dextrose rather than stopping the infusion.

The five most expensive mistakes

• Insulin before fluids.
• Insulin with a potassium below 3.3.
• Stopping the insulin infusion when glucose normalises in DKA, before the anion gap has closed.
• Choosing subcutaneous or long-acting insulin for an acute emergency.
• Correcting glucose fast and causing cerebral oedema, especially in a child.

Learn the sequence as fluids, potassium check, insulin, and these questions answer themselves regardless of which of the two conditions you are looking at.