Detective Nephron

Detective Nephron, world renowned for expert analytic skills, trains budding physician-detectives on the diagnosis and treatment of kidney diseases. Wildly waving a stack of paper records, budding nephrologist L.O. Henle and medical student Ms. Curious Tubule run down the hall toward Detective Nephron’s office.

Henle (with a smile): A case for you sir!

The detective sits facing the window. He is observing a mob outside his office with his coffee mug in hand.

Nephron (curious): Finally, something that might put an end to this utter boredom.

Henle: It’s a case of metabolic acidosis.

Nephron (smiling): Ah yes. Similar to last time! Don’t you want to give me some variety in nephrology (with a smirk).

Tubule: So this is a 50-year-old female with a history of type 2 diabetes mellitus who was admitted yesterday with left thigh abscess and eventually….

Nephron (interrupting): I don’t need any of that information. What is the bicarbonate level?

Tubule: She was found to have a serum bicarbonate of 8 mmol/L. Her sodium was 140 mmol/L, chloride was 103 mmol/L. That gives her a serum anion gap of….

Nephron (surprised look)

Tubule: . . . An anion gap of 29. So this is a high-anion gap metabolic acidosis.

Henle: Her serum pH was 7.1, and she had a PCO2 of 30 mm Hg.

Nephron: Interesting. So we have a high-anion gap metabolic acidosis and…?

Tubule: Given that she has a high-anion gap metabolic acidosis…and using Winter’s Formula would give me an expected PCO2 of 32 mm Hg. So pure high-anion gap metabolic acidosis.

Nephron: Remember there are only two body systems in my nephrocentric mind … renal and extrarenal; are we missing anything in the gap?

Tubule: Hmm, with normal anion gap being 12, her anion gap is 29. Her bicarbonate is 8 for a normal of 24. So the difference in her anion gap is close to the difference in her bicarbonate level. Hence, no other disorder exists (no additional nongap metabolic acidosis or metabolic alkalosis).

Henle (stepping in): You just told me the ΔΔ!

Nephron" ΔΔ! Who comes up with these names? Airline industry? Let’s march and rule out all causes via GOLDMARK.

Tubule (not chuckling): That was not even funny.

Nephron (laughing loudly): Nicely done. So at this point, there is just a pure high-anion gap metabolic acidosis.

Pause.

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Nephron: Hold your horses, Tubule. Can we get a urinalysis to get a sense of the urine pH?

Tubule (happy): Urine pH was 5.5.

Nephron: Good, so the kidney is dumping acid and doing its job. I assume this is a case of normal renal function?

Tubule: Yes, of course! It would be too easy for you otherwise!

Nephron: So what’s with the new GOLDMARK mnemonic?

Henle: Two popular mnemonics were used to remember the major causes of the high-gap metabolic acidoses. The first was KUSMALE, which represents ketoacidosis, uremia, salicylate poisoning, methanol, aldehyde (paraldehyde), lactate, and ethylene glycol. The second was MUD PILES, representing methanol, uremia, diabetes, paraldehyde, iron (and isoniazid), lactate, ethylene glycol, and salicylate. Metabolic acidosis due to excessive paraldehyde use has become exceedingly rare. Iron and isoniazid are just two of many drugs and toxins that cause hypotension and lactic acidosis.

Three new organic anion gap–generating acids and acid precursors have been recognized in recent years. They are D-lactic acid, which can occur in some patients with short bowel syndromes; 5-oxoproline (or pyroglutamic acid) associated with chronic acetaminophen use; and the anion gap acidosis generated by high-dose propylene glycol infusions used in lorazepam and phenobarbital drips. Therefore, recently, a newer proposal has been used to teach causes of anion gap for the 21st century: GOLDMARK. This acronym represents glycols (ethylene and propylene), oxoproline, L-lactate, D-lactate, methanol, aspirin (salicylate), renal failure, and ketoacidosis (starvation, alcohol, or diabetic).

Nephron (jumping in): Thank you for a short historical update on this! Nice job, my apprentice! Now I assume you have checked all of the above.

Tubule: Yes.

Henle (confident): She has no ingestion history and no signs of overdose of any glycols; her urine microscopy had no visualization of oxalate crystals, and she did not have toxic optic neuropathy or any neurologic findings, ruling out ethylene glycol and methanol toxicity. The psychiatry team didn’t feel she had any form of overdose. Her blood sugar is 125 mg/dL, and blood alcohol levels are negative. She has no history of any chronic use of acetaminophen, which was confirmed by a level. Her salicylate levels were negative. She was never given any medications that were prepared in propylene glycol…

Nephron: So no G, no O, no M, no A, no R, and no K? You didn’t mention anything regarding her lactic acid levels?

Henle: Yes, given no hypotension, lactic acid was normal. A D-lactate was not checked.

Tubule: I am confused. What is drug-induced lactic acid then? She must be on metformin?

Nephron: So let’s end this confusion once and for all. Not unusual to get confused. There are two types of L-lactic acidosis: type A and type B. Type A is the usual variety that you encounter in the intensive care unit with marked tissue hypoperfusion and shock state. Type B is a form of L-lactic acidosis with no apparent hypoperfusion. This is classically seen with diabetic patients on metformin and sometimes, in patients with lymphoma or other solid malignancies.

Anaerobic metabolism due to dense clusters of tumor cells and/or metastatic replacement of the hepatic parenchyma has been proposed, but lactic acidosis can develop in patients with relatively small tumor burdens. Other possible mechanisms include increased rates of lactate production by the neoplastic cells that shift to primarily aerobic glycolysis (the Warburg effect) and thiamine and/or riboflavin deficiency. It is likely that lactate metabolic clearance is also impaired. D-lactic acidosis is a rare form of lactic acidosis that can occur in patients with short bowel syndrome or other forms of gastrointestinal malabsorption. In these patients, abnormally large amounts of glucose and starch are metabolized by intestinal bacteria to multiple organic acids, including D-lactic acid. Because humans metabolize D-lactic acid very slowly, systemic absorption of the D-optical isomer of lactic acid from the bowel can lead to high plasma D-lactate levels and metabolic acidosis. She didn’t have any of those findings, I assume?

Henle: She was not on metformin.

Nephron (with a smirk): What is she taking for her diabetes? What is her A1c?

Tubule (relieved): I am not sure, but her A1c was 8.6.

Nephron: What is her urine glucose?

Henle (jumping in): Funny you ask that; it was 1000 mg/dL at multiple occasions?

Nephron: What do we think?

Tubule: With normal glucose in the serum, that is strange that she has a high urine glucose level…

Nephron: Is she on a glucoretic?

Tubule: I have heard of diuretics, aquaretics; what are glucoretics?

Henle (jumping in): He is talking about the new class of agents used to treat diabetes called sodium glucose cotransporter (SGLT-2) inhibitors. They cause increased glucose excretion via blocking this pathway in the proximal tubule.

Tubule: So what about them? What does that have to do with this acidosis?

Nephron (excitement in his eyes): In GOLDMARK, the K is ketoacidosis. It comes in three types: alcoholic, diabetic, and starvation. You told me that she has a normal alcohol level. Does she have ketoacidosis?

Henle: Hmm, her urine did have moderate ketones. Her albumin is 3.4 g/dL, and she was eating well. Doubt she has starvation.

The detective’s eyes brighten as he suddenly looks up at Ms. Tubule for a split second, then back down again.

Nephron: Fascinating.

Henle and Ms. Tubule appear puzzled.

Nephron: Please check a serum ketone level.

Tubule and Henle return a day later.

Tubule: It was high!

Henle: Why do you ask?

Nephron: So you have ruled out all causes of high-gap acidosis in this patient, but the patient has ketoacidosis clearly by urine and blood work. No starvation and no alcohol. She has diabetes and is on this novel class of agents called the SGLT-2 inhibitors. This is euglycemic diabetic ketoacidosis (eDKA); eDKA was mentioned in 1973 in the British Medical Journal in patients who were diabetic but didn’t have hyperglycemia. Compared with classic diabetic ketoacidosis (DKA), eDKA presents with mild to moderate hyperglycemia, typically <300 mg/dL blood glucose levels, which she had.

Tubule: Why is this more important now?

Nephron (continues on): In 2013, many SGLT-2 inhibitors got approved for diabetes mellitus management (the glucoretics). The Food and Drug Administration (FDA) performed an FAERS search of adverse effects with these agents, and identified 73 cases of ketoacidosis linked to SGLT-2 inhibitors. All patients required hospitalization, and 60% had DMII. Blood glucose levels ranged from 90 to 1300 mg/dL (median of 211 mg/dL). Timing of onset was around 43 days of starting or dose change of the agent. The majority of the cases also had dehydration, infection, or change in insulin doses. No mortality has been reported with this effect. All patients respond quickly with intravenous hydration and insulin once recognized.

Henle (curious): Is it a class effect?

Nephron: Yes. The initial FDA reporting was done with canagliflozin (Invokana). A more recent study found an incidence rate of 0.07% with this agent. In a large study with dapagliflozin (Farxiga), 0.1% of patients got eDKA. Empagliflozin (Jardiance) also has been found to cause eDKA.

Nephron (adds on): Dehydration, alcohol use, decrease in insulin use, infection, low-carbohydrate diet, reduction in caloric intake, and advanced age have been suggested to be risk factors for development of this entity. Apparently, she had an infection (perhaps her risk).

Henle: I don’t understand how a normal glucose level can lead to this entity?

Nephron: Ketosis results from restriction of carbohydrate usage with increased reliance on fat oxidation for energy production. The pathogenesis of hyperglycemic DKA is well understood. SGLT-2–induced glycosuria can happen over 24 hours, and this artificial low plasma glucose does not stimulate insulin. Remember, she had the high urine glucose of 1000 all along with a near-normal serum glucose. In eDKA, insulin deficiency and insulin resistance are milder; therefore, glucose overproduction and underutilization are quantitatively less than in DKA. More important, renal glucose clearance (i.e., the ratio of glycosuria to prevailing glycemia) is twice as large with eDKA than with DKA. Ketoacidosis follows with the same sequence of events in eDKA as in DKA. Insufficient insulin levels will then decrease glucose utilization and promote lipolysis and ketogenesis. In addition, these drugs can increase glucagon levels, leading to increased ketone production.

Tubule: So if I had to summarize, eDKA is pathophysiologically similar to DKA, except for the circumstance—SGLT-2–induced glycosuria—that artificially lowers plasma glucose levels and predisposes to increased ketogenesis.

Nephron: Precisely!

Henle: I just found out; she was on empagliflozin for over 3 weeks before admission.

Nephron: As I suspected.

Henle: Let me make sure I understand. To summarize, we have a patient who presented with eDKA symptomatic for 1 week, with bicarbonate initially of 8. So, obviously, she is off the drug. How do I treat?

Nephron: Hydration, and treating it similar to DKA with insulin will improve the acidosis. No data exist on a safe time to start the drug again. I wouldn’t!

Tubule: Fascinating. . .

One week later.

Tubule: Do you remember the patient we suspected of having eDKA?

Nephron: Of course.

Tubule: Right, so on our recommendation, the primary team discontinued the medication. She was given aggressive hydration and insulin treatment; eventually, acidosis resolved, and ketoacidosis disappeared. She was sent home on alternative diabetes medication.

Nephron: Very well then. And so, yet again, from a diagnosis of an acid-base disturbance, you have identified an easily reversible cause, and I hope one you will never forget. Let’s have some New York–style pizza…I am starving!

Notes

[1] Special thanks to Massini Merzkani and Holly Koncicki, both from Hofstra Northwell School of Medicine, for submitting this case. A special thanks to Helbert Rondon (Assistant Professor of Medicine, Renal-Electrolyte Division at the University of Pittsburgh School of Medicine) and Rimda Wanchoo (Professor of Medicine, Nephrology Division, Hofstra Northwell School of Medicine) for content editing.

[2] The concept of Detective Nephron was developed by Kenar D. Jhaveri, MD, Professor of Medicine at Hofstra Northwell School of Medicine and an Attending Nephrologist at Northwell Health System (NY). Send correspondence regarding this section to kjhaveri@northwell.edu or kdj200@gmail.com.