Lactic acidosis and raised anion gap

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

The anion gap is a bookkeeping estimate of negatively charged substances not routinely measured. When lactate, ketone bodies, uraemic acids or toxin metabolites accumulate, bicarbonate falls as it buffers hydrogen ions and the gap usually rises. Hypoalbuminaemia lowers the baseline gap, while chloride-rich fluid can add a simultaneous normal-gap acidosis. A patient may therefore have several processes at once.

Lactate production is not synonymous with anaerobic death. Adrenergic stimulation, seizures and high-dose beta-2 agonists can raise lactate despite adequate global oxygen delivery, whereas poor clearance in liver dysfunction prolongs it. The dangerous question is not merely why the value is high but whether shock, local ischaemia, sepsis, poisoning, DKA or organ failure is continuing.

The central decision is whether the physiology demands immediate resuscitation, whether a time-critical source such as sepsis, bowel ischaemia or toxic exposure needs definitive action, and whether the calculated gap reveals another acid when lactate does not explain the bicarbonate fall. Serial clinical response is more informative than repeatedly measuring lactate without changing treatment.

Key points

Metabolic acidosis is a low-bicarbonate process; respiratory compensation lowers carbon dioxide but never cures the underlying acid burden.
Calculate the anion gap from the local laboratory values, commonly sodium minus chloride plus bicarbonate, and interpret it against the laboratory range and serum albumin.
A low albumin can conceal clinically important unmeasured acid, so an apparently normal gap does not exclude lactate, ketones, renal acids or toxins in a critically ill patient.
Lactic acidosis describes raised lactate with acid-base disturbance, not every mildly raised lactate; trend, perfusion and the clinical cause matter more than one isolated number.
Type A hyperlactataemia reflects impaired oxygen delivery or utilisation in shock, hypoxaemia, anaemia or regional ischaemia, whereas type B causes include medicines, liver dysfunction, seizures, thiamine deficiency and malignancy.
A raised gap with normal lactate should trigger targeted testing for beta-hydroxybutyrate, renal failure, salicylate, toxic alcohols and other causes; never label every gap as lactic acidosis.
Venous blood gas usually provides rapid pH, bicarbonate and lactate information; use an arterial sample when arterial oxygenation or a precise respiratory assessment is required.
Treat perfusion and the cause: controlled crystalloid, source control and antimicrobials for sepsis, insulin-based JBDS treatment for DKA, antidote or dialysis pathways for selected toxins, and surgery for ischaemic bowel.
Sodium bicarbonate is not routine treatment for lactic acidosis and can add sodium, carbon dioxide and volume; reserve it for defined specialist-led indications under the current local critical-care or toxicology protocol.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Tissue hypoperfusion

Shock, sepsis, severe hypoxaemia or local ischaemia limits oxidative metabolism and increases lactate production in threatened tissues.

Adrenergic or muscular production

Seizures, intense beta-adrenergic stimulation and high-dose beta-agonists can raise lactate despite adequate global oxygen delivery, particularly while muscular or adrenergic activity continues.

Reduced hepatic clearance

Severe liver dysfunction prolongs lactate elevation by limiting metabolism, often alongside increased production from the underlying critical illness.

Other unmeasured acids

Ketones, uraemic acids and toxin metabolites can raise the anion gap with or without lactate, producing mixed acid-base disease.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Organic acids accumulate
Lactate or another unmeasured anion rises when production exceeds metabolism and renal or hepatic clearance as the process continues.
2
Bicarbonate buffers hydrogen ions
Buffering consumes bicarbonate, lowering pH and usually widening the calculated anion gap as the accompanying anion remains unmeasured.
3
Ventilation compensates
Hydrogen-ion sensing increases respiratory drive and lowers carbon dioxide, but fatigue, sedation or lung disease can make compensation fail.
4
Acidaemia worsens organ function
Severe acidaemia impairs myocardial contractility, vascular responsiveness and cellular enzyme function, amplifying shock and organ failure.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Shock-associated lactateRed flag

Hypotension, mottling, prolonged capillary refill, cool peripheries, confusion and oliguria with rising lactate indicate threatened organ perfusion even when a single blood pressure appears acceptable.

Compensatory breathingRed flag

Deep rapid respiration may be physiological compensation for metabolic acidosis. Sedation or exhaustion can remove compensation and cause abrupt pH deterioration, so airway decisions require senior critical-care planning.

Regional ischaemiaRed flag

Severe abdominal pain out of proportion, gastrointestinal bleeding, limb pain with absent pulses or evolving compartment signs can generate lactate before global shock becomes obvious.

Ketoacid accumulationRed flag

Vomiting, dehydration, abdominal pain, diabetes, starvation, pregnancy or heavy alcohol use with raised blood beta-hydroxybutyrate suggests ketoacidosis; glucose may be modest with SGLT2 exposure or pregnancy.

Metformin accumulationRed flag

Severe illness, acute kidney injury, hypoxia or shock in a person taking metformin can permit accumulation and lactic acidosis, but clinicians must still find and treat the precipitating disease.

Toxic alcohol patternRed flag

Visual disturbance, inebriation, renal injury, hypocalcaemia or unexplained high osmolar and anion gaps suggests methanol or ethylene glycol exposure and requires immediate poisons and renal advice.

Post-seizure rise

A generalised convulsion can cause a brisk transient lactate increase that should fall with recovery; persistence suggests ongoing seizure, shock, sepsis or another source.

Drug-related adrenergic rise

High-dose salbutamol or adrenaline can raise lactate and worsen tachypnoea despite improving bronchospasm; reassess airways and ventilation before escalating bronchodilator solely because breathing remains fast.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Venous or arterial blood gasFirst step
Why
Confirm acidemia, bicarbonate reduction, respiratory compensation and current lactate rapidly.
Interpretation and limitations
Interpret pH, carbon dioxide and bicarbonate together. A normal pH can conceal mixed acidosis and alkalosis; compare measured compensation with the clinical respiratory state and use arterial sampling when oxygenation is uncertain.
02
Serum electrolytes and calculated anion gap
Why
Detect unmeasured acid and concurrent renal or electrolyte danger.
Interpretation and limitations
Use the local equation and reference range. Review sodium, chloride and bicarbonate, account for albumin when available and recognise that chloride-rich resuscitation can produce a simultaneous normal-gap process.
03
Serial blood lactate
Why
Measure trajectory after resuscitation and definitive treatment.
Interpretation and limitations
A falling value supports improving production or clearance, but must match perfusion and organ function. Failure to fall should prompt reassessment for inadequate source control, regional ischaemia, seizure, medicine effect or impaired clearance.
04
Capillary glucose and blood beta-hydroxybutyrate
Why
Identify diabetic, starvation, alcoholic or pregnancy-associated ketoacidosis.
Interpretation and limitations
Blood ketones are preferred to urine for current severity. Apply the current JBDS DKA pathway when diagnostic criteria are met; near-normal glucose does not exclude SGLT2-associated or pregnancy DKA.
05
Renal profile, liver tests and full blood count
Why
Assess acid clearance, metformin risk, hepatic lactate clearance, anaemia and infection.
Interpretation and limitations
Acute creatinine rise, severe anaemia or liver failure can drive or prolong acidosis and alters medicine and fluid decisions; trend from baseline rather than attributing everything to dehydration.
06
Serum osmolality and osmolar gap
Why
Support assessment of suspected toxic alcohol or other osmotically active exposure.
Interpretation and limitations
Calculate with the locally validated formula and contact the National Poisons Information Service early. A normal osmolar gap later in poisoning does not exclude toxic metabolites once the parent alcohol has been consumed.
07
Salicylate and toxicology-directed assays
Why
Detect salicylate or another exposure when history or mixed acid-base findings suggest poisoning.
Interpretation and limitations
Salicylate commonly produces respiratory alkalosis with metabolic acidosis; repeat concentrations and blood gases under poisons guidance because a single early level may be misleading.
08
Cultures and source imaging
Why
Find infection, tissue necrosis, bowel ischaemia or another reversible driver.
Interpretation and limitations
Obtain cultures without delaying indicated antimicrobials and select imaging from the suspected source. A normal early lactate does not safely exclude mesenteric ischaemia when the clinical picture is concerning.
09
ECG and continuous physiological monitoring
Why
Detect hyperkalaemia, sodium-channel toxicity, ischaemia and deterioration during correction.
Interpretation and limitations
Treat malignant rhythm or severe potassium changes immediately. QT, QRS and perfusion trends may reveal toxin or electrolyte consequences before a repeat chemistry result returns.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Diabetic ketoacidosis

Raised blood ketones and insulin-deficient diabetes explain a high gap even when lactate is also modestly elevated through dehydration or stress.

Uraemic acidosis

Advanced kidney failure with retained acids and declining filtration supports uraemia rather than isolated lactate overproduction.

Toxic alcohol or salicylate exposure

Exposure history, visual or neurological features, osmolal assessment and targeted toxicology distinguish poisoning from ordinary critical-illness lactate.

Hyperchloraemic acidosis

Bicarbonate loss or chloride-rich fluid lowers bicarbonate without widening the gap and may coexist after treatment of a high-gap process.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01ResuscitateShock with hyperlactataemiaFirst stepRaised lactate accompanied by hypotension, hypoperfusion, altered consciousness or organ dysfunction.

1Start ABCDE, high-acuity monitoring, intravenous access and urgent blood gas, glucose, ketones, electrolytes and cultures while calling senior and critical-care support.
2Give controlled crystalloid boluses with repeated response assessment, oxygen only to the appropriate target, and early vasopressor support through critical care when shock persists.
3Treat the cause in parallel: follow NICE sepsis timing and source control, control haemorrhage, restore flow to an ischaemic limb or bowel, and correct severe hypoxaemia or anaemia as indicated.
4EscalationTrend lactate, pH, urine output, mental state and peripheral perfusion, stopping ineffective fluid loading and escalating when physiology or organ function fails to improve.

02GapRaised anion gap without explanatory lactateBicarbonate is low and the calculated gap is raised, but lactate cannot account for the disturbance.

1Confirm the result, review albumin and chloride, compare the gas bicarbonate with laboratory chemistry and look for a mixed respiratory or normal-gap component.
2Measure blood beta-hydroxybutyrate, renal function, glucose and salicylate, and obtain osmolality when toxic alcohol exposure is plausible.
3Use history, medicines, pregnancy status, alcohol, fasting, diabetes technology and occupational exposure to target additional tests rather than ordering an indiscriminate toxicology screen.
4ConfirmatoryStart the relevant emergency pathway immediately for DKA, salicylate or toxic alcohol poisoning, renal failure or starvation and do not wait for every confirmatory result.

03MetforminPossible metformin-associated lactic acidosisMarked acidosis and lactate in a metformin user with renal failure, hypoxia, shock or overdose.

1Stop metformin, stabilise ABCDE and measure pH, serial lactate, renal function, glucose, ketones and coexisting sepsis or hypoperfusion; metformin may be contributor rather than sole cause.
2Contact critical care, renal medicine and the National Poisons Information Service early; use current toxicology criteria to decide extracorporeal treatment rather than a remembered lactate threshold alone.
3Correct the precipitant and fluid or haemodynamic deficit carefully, monitoring potassium, glucose, temperature, urine output and ventilation as acidosis changes.
4Restart metformin only after the acute cause and renal function have recovered and the licensed eligibility remains satisfied, with explicit future sick-day advice.

04PersistingLactate not clearing as expectedLactate remains elevated or rises after apparently adequate initial treatment.

1Re-examine perfusion, fluid responsiveness, oxygen delivery, haemoglobin, infection source and vasopressor needs rather than ordering repeat lactate in isolation.
2Search actively for regional ischaemia, occult seizure, thiamine deficiency, liver dysfunction, malignancy and adrenergic medicine exposure.
3Review sampling quality and timing while assuming the result is real until clinical reconciliation; avoid dismissing deterioration as laboratory error.
4EscalationEscalate source control, imaging, surgery, toxicology or organ support and define the next physiological review point with a named senior clinician.

Key medicines and prescribing safety5 treatments · regimens, roles and cautions

Restores effective circulating volume and tissue perfusion when hypovolaemia contributes to type A lactate production.

Balanced isotonic crystalloid

Give cautious protocol-led boluses with bedside reassessment after each aliquot rather than an automatic large volume.

Pulmonary oedema, cardiac or renal failure and absent fluid responsiveness require smaller challenges and earlier vasoactive support; excessive chloride-containing fluid can worsen acidosis.

Treats bacterial infection driving distributive shock and lactate accumulation while source control is arranged.

Empirical antimicrobial therapy

Administer the current local sepsis regimen at the NICE-defined urgency after appropriate cultures when feasible.

Choice depends on source, allergy, renal function, pregnancy and resistance; do not delay treatment in high-risk sepsis while seeking an ideal sample.

Restores an essential cofactor for oxidative metabolism when deficiency contributes to persistent lactate.

Intravenous thiamine

Use the local high-risk alcohol or suspected deficiency regimen before or alongside carbohydrate and nutrition.

Do not use a response to thiamine as proof of diagnosis or let administration delay resuscitation; parenteral preparations require monitoring for rare hypersensitivity.

Stops ketogenesis and clears beta-hydroxybutyrate when ketoacidosis explains the raised gap.

Fixed-rate intravenous insulin for DKA

Prescribe through the current JBDS DKA chart with concurrent fluids, potassium surveillance and continued basal insulin.

It is not generic treatment for lactic acidosis. Prevent hypoglycaemia, hypokalaemia and fluid harm and use pregnancy or paediatric protocols where applicable.

Provides temporary alkali in selected situations while definitive cause treatment or extracorporeal support is organised.

Sodium bicarbonate

Use only the specialist-selected dose and endpoint for a defined severe acidemia or poisoning indication.

Routine use in lactic acidosis can cause sodium and volume load, carbon dioxide generation, reduced ionised calcium and paradoxical intracellular acidosis; local critical-care or poisons guidance is mandatory.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Refractory shock

Severe acidaemia reduces cardiac performance and catecholamine responsiveness, while continuing tissue hypoperfusion generates more lactate and perpetuates shock.

Cardiac arrhythmia

Severe acidaemia and associated potassium disturbance destabilise myocardial conduction, reducing cardiac performance and potentially progressing to cardiac arrest.

Respiratory exhaustion

Sustained compensatory hyperventilation increases work of breathing and may fail abruptly in fatigue, sedation or respiratory disease.

Progressive multiorgan injury

Ongoing sepsis, ischaemia, poisoning or organ failure damages kidney, liver, brain and gut while acid clearance deteriorates.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat ABCDE observations, capillary refill, mental state, temperature and urine output frequently; lactate is one component of perfusion assessment rather than the resuscitation target alone.
Trend pH, bicarbonate, lactate, potassium, glucose, ketones and creatinine at an interval matched to severity and the active emergency pathway.
Reassess after every fluid bolus for benefit and congestion, moving to vasopressor, inotrope or definitive source control when further fluid is unlikely to help.
Use continuous ECG and oxygen monitoring in severe acidosis, potassium disturbance, suspected poisoning or vasoactive treatment, with critical-care review for respiratory fatigue.
Document medicine interruption and explicit restart criteria for metformin, SGLT2 inhibitors and nephrotoxic or haemodynamically active agents after recovery.
After resolution, identify the precipitant, reconcile alcohol or toxin risk, arrange renal or diabetes follow-up and provide personalised sick-day advice to prevent recurrence.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Correct the gap context

Albumin is a major unmeasured anion. A low concentration narrows the baseline gap and can disguise substantial lactate or ketoacid accumulation.

Lactate is not perfusion alone

Adrenergic glycolysis, seizure and impaired hepatic clearance can raise lactate without global tissue hypoxia; interpret the whole circulation and trend.

Compensation can fail

A patient maintaining pH through deep ventilation may deteriorate rapidly with fatigue, opiates or intubation. Airway management must preserve minute ventilation while definitive therapy starts.

One gap can hide another

Vomiting can add metabolic alkalosis and chloride-rich fluid can add normal-gap acidosis. A near-normal pH does not mean the acid burden is minor.

Normal lactate does not clear bowel

Mesenteric ischaemia is a clinical and imaging diagnosis; lactate may rise late and a reassuring early result must not overrule disproportionate pain.

Dialysis treats selected causes

Metformin accumulation, toxic alcohols, salicylate and severe renal failure have different extracorporeal criteria. Early renal and poisons advice prevents dangerous threshold guessing.

Treat the patient, not clearance

Repeated fluid solely to force lactate down can cause oedema while shock remains vasoplegic or the source remains uncontrolled. Each intervention needs a physiological rationale.

11Common pitfallsFrequent interpretation and management errors.

01
Equating every elevated lactate with septic shock.
02
Calling a normal uncorrected anion gap reassuring in severe hypoalbuminaemia.
03
Missing ketoacidosis because glucose is not markedly elevated.
04
Using sodium bicarbonate routinely instead of treating perfusion and cause.
05
Giving repeated fluid despite pulmonary congestion and absent response.
06
Attributing all acidosis in a metformin user to the medicine and missing bowel ischaemia or sepsis.
07
Excluding toxic alcohol ingestion because the osmolar gap is normal late in presentation.
08
Allowing intubation or sedation to remove compensatory ventilation without an acidemia plan.