The QT Interval and the Electrolytes: The Evidence Behind Two Tools

A 62-year-old farmer from Shivamogga is admitted with three days of watery diarrhoea. He takes hydrochlorothiazide for his blood pressure. On the way in, he was given ondansetron for the vomiting, and he has been on levosulpiride for “gas” for months. His potassium is 2.9 mEq/L and his magnesium 1.3 mg/dL. The ECG machine prints a QTc of 528 ms at a heart rate of 112. The intern asks two questions at once. Is the QT really that long? And which do I correct first? This is an illustrative case, not a real patient.

Two MEDiscuss tools answer those questions at the bedside. The QTc calculator corrects the interval five ways and tells you which formula the heart rate allows. The electrolyte panel puts the corrections in order and doses them. What the two tools no longer carry is some of the why: the theory behind the formulas, the studies behind the numbers, and the mechanisms. Those have moved here, so that the tools stay quick on the ward. By the end of this article you will know why the machine’s QTc can mislead you, and why magnesium comes first.

1. Why there are five QT formulas, not one

Start with the number on the ECG. Repolarisation shortens as the heart speeds up. So the raw QT must be corrected to what it would have been at 60 beats per minute. Only then can two ECGs be compared.

“Then why does every textbook give a different formula?” Because the correction is an empirical fit to population data, not a physiological law. That is why five formulas exist, and none is right for everybody. All five agree exactly at a rate of 60. There the RR interval is one second, and every formula returns the measured QT. Either side of 60 they diverge. At the extremes they differ by more than 50 ms, which is the width of the whole decision.

Our patient’s rate is 112. The machine almost certainly used Bazett, which over-corrects at that rate. The calculator leads with Fridericia, prints all five, and shows you how far apart they sit. That gap is the error you would have carried had you trusted the printout. Measure the QT by hand, in lead II or V5, before you believe 528.

The common mistake is a natural one. We read the machine’s QTc as a measurement. It is a calculation on a measurement, by a formula chosen for you.

2. QT monitoring in Indian tuberculosis care

Drug-resistant tuberculosis regimens stack QT-prolonging drugs for months: bedaquiline, delamanid, moxifloxacin and clofazimine. The national programme therefore asks for regular ECGs, and the QTc calculator carries its schedule and its four grades.

“How often does the QT actually become dangerous on these drugs?” Indian data give a reassuring answer. Among 70 patients in Mumbai given bedaquiline and delamanid concomitantly under programmatic conditions, QTcF prolongation was recorded in about 7 per cent. Only two exceeded 500 ms, both within the first month, and neither subsequently. The rate was lower than in comparable series (Das and colleagues, reference 13 of the first list). It argues for concentrating a district programme’s scarce ECG capacity in the first four to eight weeks. It does not argue for skipping the baseline ECG.

3. Back to our patient: magnesium before potassium

The QT is long, and his potassium and magnesium are low. The electrolyte panel will tell the intern to give magnesium first. Here is why.

Potassium leaves the distal nephron through the ROMK channel. That channel is held partly shut by intracellular magnesium, plugging its pore from the cytosolic side. Normal intracellular free magnesium, 0.5 to 1.0 mmol/L, sits on the steep part of that inhibition curve. Deplete the magnesium and the plug comes out. ROMK opens, and the kidney wastes potassium however much is given.

Think of a leaking bucket. You can pour potassium in all day. Until the magnesium is back, the hole stays open.

“How often is the magnesium low as well?” Often. Huang and Kuo estimate that more than half of clinically significant hypokalaemia carries a concurrent magnesium deficiency. Our farmer has two of the usual Indian causes, a diuretic and diarrhoea. So the panel puts magnesium first, then potassium, and it counts the rise in sodium that potassium replacement brings.

4. The adjusted calcium, and why it misleads

His calcium comes back at 7.6 mg/dL with an albumin of 2.6 g/dL. The ward habit is to “adjust” it with the Payne formula. The panel prints the formula and warns against trusting it where it matters most.

“Surely correcting for albumin is better than nothing?” The studies say not. In one prospective ICU cohort, the adjusted calcium identified 15.2 per cent of truly hypocalcaemic patients, against 62.8 per cent for the uncorrected total calcium. Albumin added nothing once total calcium, pH and sex were in the model. In patients with an albumin below 30 g/L, three quarters of those called normocalcaemic by the formula were in fact hypocalcaemic.

So the formula is weakest in exactly the patient you use it on, the sick one with a low albumin. Where the decision turns on the calcium, measure the ionised calcium on a blood gas analyser.

5. Why the panel does not use the TTKG

Many of us were taught the transtubular potassium gradient for working up hypokalaemia. The panel uses the spot urine potassium to creatinine ratio instead.

“Why drop a test that is still in the books?” Because the people who devised it withdrew it. Kamel and Halperin showed in 2011 that its central assumption is invalid. The TTKG assumes that no appreciable osmoles are reabsorbed downstream of the cortical collecting duct. In fact a large quantity of urea is recycled in the inner medullary collecting duct. In 2021 they wrote plainly that they do not use the TTKG in assessing patients with potassium disorders.

6. The reference ranges, in one table

The panel shows its ranges and severity bands as two narrow tables, so they fit a phone. Here they are as one table, as the tool carried them on 4 November 2026. Take the anion gap range from your own laboratory’s report.

Back to the ward

The intern measured the QT by hand and corrected it by Fridericia: still long, but less than the machine said. Ondansetron and levosulpiride were stopped, and the thiazide held. Magnesium sulphate went in first, then potassium chloride, with the sodium watched. The next morning his potassium was holding at last, and the QTc was shorter. Each of those steps is a line in one of the two tools.

Try this

A 45-year-old woman on furosemide has a potassium of 3.0 mEq/L that has not moved after two days of oral potassium chloride. Her ECG is normal. What do you send next, and why? Illustrative case.

Answer. Send a serum magnesium. A hypokalaemia that will not correct is a magnesium level you have not sent. With magnesium low, ROMK stays open and the kidney keeps wasting the potassium you give. Replace the magnesium, and the potassium usually follows. If you still need to know why she is losing potassium, use the spot urine potassium to creatinine ratio, not the TTKG.

Take-home points

• The machine’s QTc is a formula applied to a measurement; at a fast or slow rate, check it by hand and by Fridericia.
• All five QT formulas agree at a rate of 60 and part company either side of it.
• Correct magnesium before potassium: without magnesium, the kidney keeps wasting potassium.
• In a sick patient with a low albumin, measure the ionised calcium; do not trust the adjusted value.
• The TTKG has been withdrawn by its authors; use the urine potassium to creatinine ratio.

Declaration: the author built and maintains both tools. They are free to use. The figures in this article are those the tools carried in their Evidence tabs on 4 November 2026, with the sources as the tools named them. The illustrative case follows the tools’ rules; check each tool’s result for a real patient.

References

Corrected QT (QTc) Calculator

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Combined Electrolyte Panel

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