When a patient’s ABG reveals metabolic acidosis with a normal anion gap, the next step is figuring out why. The HARDUPS mnemonic organizes the seven most important causes into a single word you can recall at the bedside — and pairs directly with MUDPILES, which covers elevated anion gap causes.
HARDUPS stands for: Hyperalimentation, Acetazolamide, Renal tubular acidosis, Diarrhea, Ureteroenterostomy, Pancreatic fistula / Post-hypocapnia, Saline infusion / Spironolactone.
Unlike elevated anion gap acidosis, where an unmeasured anion accumulates in the blood, normal anion gap (hyperchloremic) metabolic acidosis results from bicarbonate loss or failure to excrete hydrogen ions — with chloride rising to maintain electrical neutrality. The anion gap stays within normal range (8–12 mEq/L) because no new unmeasured anion enters the equation.
The HARDUPS mnemonic
| Letter | Cause |
|---|---|
| H | Hyperalimentation (TPN) |
| A | Acetazolamide (carbonic anhydrase inhibitor) |
| R | Renal tubular acidosis (types I, II, IV) |
| D | Diarrhea (GI bicarbonate loss) |
| U | Ureteroenterostomy / urinary diversion |
| P | Pancreatic fistula / Post-hypocapnia |
| S | Saline infusion (dilutional) / Spironolactone |
Normal anion gap vs. elevated anion gap: key differences
The anion gap (AG) formula is:
AG = Na⁺ − (Cl⁻ + HCO₃⁻)
Normal range: 8–12 mEq/L.
| Feature | Normal anion gap acidosis (HARDUPS) | Elevated anion gap acidosis (MUDPILES) |
|---|---|---|
| Anion gap | 8–12 mEq/L | > 12 mEq/L |
| Mechanism | Bicarbonate loss or H⁺ retention without new unmeasured anion | Accumulation of unmeasured organic or inorganic anion |
| Chloride | Elevated (hyperchloremic) | Normal or low |
| Common causes | Diarrhea, RTA, saline infusion | DKA, lactic acidosis, salicylates, renal failure |
| Urinary pH clue | Variable — helps distinguish RTA types | Less diagnostically useful |
Understanding this distinction is essential for ABG interpretation. After confirming metabolic acidosis and calculating the AG, the gap value determines which mnemonic you reach for.
Detailed breakdown of each cause
H — Hyperalimentation (TPN)
Total parenteral nutrition (TPN) is intravenous nutrition delivered through a central line when a patient cannot absorb nutrients via the GI tract. Hyperalimentation-induced metabolic acidosis develops because many amino acids in TPN formulations — particularly arginine, lysine, and histidine — are positively charged. When these amino acids are metabolized, they release hydrogen ions. If the TPN formula does not include sufficient bicarbonate precursors (such as acetate or lactate), those hydrogen ions accumulate and consume bicarbonate.
Why the anion gap stays normal: The metabolized amino acids do not leave a measurable organic anion behind. The net effect is HCO₃⁻ consumption paired with a compensatory rise in chloride.
Clinical context: ICU patients on long-term TPN are the primary population. Nurses should monitor serum bicarbonate trends and electrolyte panels during TPN administration. TPN formulations can be adjusted by pharmacy to increase acetate content and reduce chloride load when acidosis develops.
Key labs: Decreased HCO₃⁻ (< 22 mEq/L), normal AG, hyperchloremia. pH < 7.35 if uncompensated.
A — Acetazolamide
Acetazolamide (Diamox) is a carbonic anhydrase inhibitor used for glaucoma, altitude sickness, some seizure types, and as a diuretic. Carbonic anhydrase is the enzyme that catalyzes the conversion of CO₂ and water into carbonic acid, which then dissociates into H⁺ and HCO₃⁻ in the renal tubule. By blocking this enzyme in the proximal tubule, acetazolamide prevents bicarbonate reabsorption, causing large amounts of HCO₃⁻ to spill into the urine.
Why the anion gap stays normal: Bicarbonate is lost directly in the urine. Chloride rises in compensation; no organic acid accumulates.
Clinical context: Acetazolamide-induced acidosis is dose-dependent and predictable. It is effectively a pharmacologically induced proximal renal tubular acidosis (Type 2 RTA). Nurses administering acetazolamide chronically should monitor serum electrolytes and bicarbonate regularly. The effect reverses when the drug is stopped.
Key labs: Low HCO₃⁻, low urinary pH early (bicarbonate loss makes urine alkaline initially, then acidic as stores deplete), hyperchloremia.
R — Renal tubular acidosis
Renal tubular acidosis (RTA) is a group of disorders where the kidney fails to maintain acid-base balance despite adequate glomerular filtration. There are three clinically relevant types:
Type 1 (distal RTA): The distal nephron cannot secrete H⁺ into the collecting tubule. Result: urine remains alkaline (pH > 5.5) despite systemic acidosis. Causes include autoimmune diseases (Sjögren’s syndrome, lupus), medications (amphotericin B), and genetic disorders. Hypokalemia is a hallmark — the kidney compensates for impaired H⁺ secretion by excreting more K⁺.
Type 2 (proximal RTA): The proximal tubule fails to reabsorb bicarbonate, allowing it to spill into the urine. Associated with Fanconi syndrome, multiple myeloma, heavy metal toxicity, and carbonic anhydrase inhibitors. Also presents with hypokalemia. Urine pH may be acidic (< 5.5) late in the process once plasma bicarbonate falls below the reabsorption threshold.
Type 4 (hyperkalemic RTA): Hypoaldosteronism or aldosterone resistance reduces H⁺ and K⁺ secretion in the collecting duct. Unlike types 1 and 2, this type presents with hyperkalemia — an important distinguishing feature. Common causes include diabetic nephropathy, adrenal insufficiency, and medications such as ACE inhibitors, ARBs, and potassium-sparing diuretics.
Nursing relevance: Potassium levels are a key differentiator. Types 1 and 2 RTA present with hypokalemia; Type 4 presents with hyperkalemia. Electrolyte imbalances secondary to RTA can be clinically significant. Monitor potassium closely and expect replacement therapy with Types 1 and 2.
D — Diarrhea
Diarrhea is one of the most common causes of normal anion gap metabolic acidosis in both inpatient and outpatient settings. The mechanism is straightforward: the small intestine and colon secrete bicarbonate-rich fluid into the gut lumen. Normally this fluid is reabsorbed, but in diarrhea, increased gut motility flushes it out before reabsorption can occur.
Why the anion gap stays normal: The loss is of bicarbonate, not the accumulation of acid. As HCO₃⁻ falls, the kidneys retain chloride to preserve electroneutrality, producing hyperchloremia with a preserved AG.
Clinical context: Severe or prolonged diarrhea — from infections, inflammatory bowel disease, or osmotic causes — can produce significant acidosis. Infants and elderly patients are particularly vulnerable to rapid bicarbonate depletion. Nurses should monitor electrolytes (especially K⁺, which is also lost in stool) and assess hydration status. Oral or IV rehydration solutions containing bicarbonate precursors are part of management.
Key labs: Low HCO₃⁻, low K⁺, normal AG, hyperchloremia. Stool electrolyte analysis may show a negative fecal osmotic gap.
U — Ureteroenterostomy / urinary diversion
Ureteroenterostomy (also called ureterosigmoidostomy or ileal conduit) is a surgical urinary diversion procedure in which ureters are implanted into a segment of bowel, typically after cystectomy for bladder cancer. When urine contacts intestinal mucosa, two acidifying processes occur:
- Chloride-bicarbonate exchange: The gut actively absorbs Cl⁻ from urine while secreting HCO₃⁻ into the lumen, which is then lost in stool.
- Ammonium absorption: The bowel absorbs NH₄⁺ from urine, which is metabolized to H⁺ + NH₃, generating an acid load.
Why the anion gap stays normal: Bicarbonate is lost or consumed without generating an unmeasured anion.
Clinical context: Patients with urinary diversions require ongoing monitoring for chronic metabolic acidosis. The longer the bowel segment in contact with urine, and the longer the urine dwell time, the greater the acid load. Ileal conduit patients typically have less contact time (draining freely) and milder acidosis than ureterosigmoidostomy patients. Nurses caring for post-cystectomy patients should include serum bicarbonate in routine labs and watch for fatigue, altered mental status, or compensatory tachypnea.
P — Pancreatic fistula / Post-hypocapnia
This letter covers two separate mechanisms:
Pancreatic fistula: Pancreatic juice has a high bicarbonate concentration — up to 120–140 mEq/L in high-output secretion. When a pancreatic fistula (an abnormal connection between the pancreatic duct and body cavity or skin) allows pancreatic secretions to drain externally or into the peritoneum, the body loses massive quantities of bicarbonate. This presents similarly to severe diarrhea but is often seen post-operatively or in patients with severe pancreatitis.
Post-hypocapnia: This is a transitional state, not a primary disease. When chronic respiratory alkalosis (low PaCO₂, low CO₂) has been present for some time, the kidneys compensate by excreting bicarbonate to normalize pH. If the respiratory alkalosis is then suddenly corrected — for example, when a patient on mechanical ventilation is rapidly weaned — CO₂ returns to normal, but the kidneys have already depleted bicarbonate. The result is transient normal anion gap metabolic acidosis until bicarbonate is regenerated.
Clinical context for post-hypocapnia: Most commonly seen in mechanically ventilated patients. Nurses should anticipate this during ventilator weaning and communicate with the team about gradual versus rapid ventilator changes.
S — Saline infusion / Spironolactone
Two separate causes share this letter:
Saline infusion (dilutional acidosis): Large-volume infusion of normal saline (0.9% NaCl) delivers a substantial chloride load. Chloride and bicarbonate compete for tubular transport and electrical balance — as chloride rises sharply, bicarbonate is suppressed and partially excreted to maintain electroneutrality. This is sometimes called “hyperchloremic dilutional acidosis” and is commonly seen in surgical or trauma patients receiving aggressive saline resuscitation.
The mechanism is not only dilutional: the high chloride concentration directly inhibits bicarbonate reabsorption in the renal tubule. Balanced crystalloids (Lactated Ringer’s, Plasma-Lyte) are associated with significantly lower rates of hyperchloremic acidosis compared to normal saline, which is why many critical care protocols have shifted away from large-volume saline resuscitation.
Spironolactone: This potassium-sparing diuretic works by blocking aldosterone receptors in the distal tubule. Aldosterone normally drives both K⁺ excretion and H⁺ secretion. When aldosterone is blocked, the collecting duct cannot excrete H⁺ normally, causing hydrogen ion accumulation — the same mechanism as Type 4 RTA. Spironolactone-induced metabolic acidosis presents with hyperkalemia, mirroring aldosterone deficiency.
Key labs (saline): Hyperchloremia, low-to-normal HCO₃⁻, normal AG. Serum chloride often > 110 mEq/L.
Key labs (spironolactone): Hyperkalemia, low HCO₃⁻, normal AG.
Clinical application: using HARDUPS in practice
HARDUPS fits into ABG interpretation at the step where you have already confirmed metabolic acidosis and calculated the anion gap. The workflow is:
- Confirm pH < 7.35 → acidosis
- Confirm HCO₃⁻ < 22 mEq/L → metabolic component
- Calculate AG = Na⁺ − (Cl⁻ + HCO₃⁻)
- AG ≤ 12? → Use HARDUPS to identify the cause
- AG > 12? → Use MUDPILES to identify the cause
- Assess ROME to confirm compensation direction (respiratory alkalosis should compensate for metabolic acidosis)
The ROME mnemonic — Respiratory Opposite, Metabolic Equal — helps confirm that the compensatory response matches expectations. In metabolic acidosis, expect respiratory compensation: the pH and HCO₃⁻ move in the same (Equal) direction. Normal compensation is a drop in PaCO₂ of approximately 1–1.5 mmHg for each 1 mEq/L fall in HCO₃⁻ (Winter’s formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2).
Context clues that point to HARDUPS causes
| Clinical clue | Likely cause |
|---|---|
| Long-term TPN, ICU patient | Hyperalimentation |
| Glaucoma or altitude sickness medications | Acetazolamide |
| Hypokalemia + alkaline urine + no diarrhea | Distal or proximal RTA |
| Hyperkalemia + low aldosterone or ACE inhibitor use | Type 4 RTA / Spironolactone |
| Profuse watery diarrhea | Diarrhea |
| Post-cystectomy, ileal conduit | Ureteroenterostomy |
| Acute pancreatitis or post-operative drain output | Pancreatic fistula |
| Large-volume normal saline resuscitation | Saline infusion |
| Mechanical ventilation weaning | Post-hypocapnia |
Common mistakes nursing students make
Confusing AG-normal and AG-elevated acidosis. Calculate the AG before reaching for a mnemonic. Skipping the calculation leads to applying MUDPILES when HARDUPS is needed — or missing a mixed picture entirely.
Forgetting that RTA comes in three types with different potassium levels. Types 1 and 2 cause hypokalemia; Type 4 causes hyperkalemia. This is a high-yield NCLEX distinction and a clinically important nursing assessment point.
Missing the saline connection. Students often overlook that aggressive IV saline is an iatrogenic cause of normal anion gap acidosis. When a post-op patient develops mild hyperchloremic acidosis after receiving several liters of 0.9% saline, the fluid — not the patient’s disease — may be the cause.
Not using HARDUPS and MUDPILES together. Some patients have a mixed-gap picture. If the AG is elevated but not as elevated as expected for the degree of HCO₃⁻ loss (delta-delta ratio), a concurrent normal-gap process may be present.
Related mnemonics and study tools
- MUDPILES mnemonic — the complementary mnemonic for elevated anion gap metabolic acidosis
- ROME mnemonic — determines whether the primary disorder is respiratory or metabolic, and confirms compensation direction
- ABG interpretation guide — full 5-step systematic method with normal values, compensation formulas, and worked examples
- Electrolyte imbalances in nursing — potassium, chloride, and bicarbonate abnormalities in clinical context
Summary
HARDUPS gives nursing students a reliable framework for the seven main causes of normal anion gap (hyperchloremic) metabolic acidosis. Each cause reduces bicarbonate — either by GI or urinary loss, by impairing renal H⁺ excretion, or by delivering an exogenous acid or chloride load. The anion gap stays normal because no unmeasured anion accumulates.
When your patient’s ABG shows metabolic acidosis with a normal AG, work through HARDUPS systematically. Consider the patient’s medications (acetazolamide, spironolactone, recent saline), recent GI history (diarrhea, fistula output), surgical history (urinary diversion), and whether they are receiving TPN or are post-ventilation. Most causes will surface with a focused history and basic labs.
Pair HARDUPS with MUDPILES and ROME for a complete ABG interpretation toolkit.