The anion gap helps organise acid-base interpretation, but the number only makes sense alongside the reporting laboratory's reference interval, albumin and the wider clinical picture. Use the calculator for the arithmetic; use this Guide for what a raised, normal, low or negative gap can mean.
Start with the equation and the local range
MedMaths uses the common potassium-free equation. The gap reflects the balance between unmeasured anions and unmeasured cations; it is not a direct measurement of one substance. Because analyser methods and equation conventions differ, there is no single universal reference interval. Interpret the result against the range used by the reporting laboratory.
Why albumin matters
Albumin is an important unmeasured anion. When albumin is low, the observed gap can also fall, potentially making another increase in unmeasured anions less obvious. Figge and colleaguesquantified an albumin effect of roughly 2.5 mmol/L per 1 g/dL change in albumin.
Albumin adjustment is an estimate, not a universal laboratory standard. Published literature supports an albumin effect, but the clinical value of routinely applying a correction formula is debated. Keep the standard gap and measured albumin visible and use the convention required by the relevant laboratory or source. Albumin can be reported in g/dL or g/L, so verify the unit before applying any adjustment.
High anion gap: GOLDMARK
A gap above the local reference interval suggests an increase in unmeasured anions. A commonly used differential framework is GOLDMARK:
GOLDMARK is a differential framework, not a diagnosis. The gap does not identify which cause is present. Lactate, ketones, renal studies, toxicology testing, medication/exposure history and blood-gas assessment may be needed depending on the clinical situation.
Anion gap in DKA: useful clue, not the primary diagnostic or resolution test
Current diabetic ketoacidosis guidance is more specific than simply looking for a raised anion gap. The 2026 American Diabetes Association criteria require diabetes or hyperglycaemia, ketosis and metabolic acidosis. Anion gap is not one of the required diagnostic criteria.
The 2024 international hyperglycaemic-crisis consensus recommends direct blood beta-hydroxybutyratefor DKA diagnosis and monitoring when available. Mixed acid-base disorders are common, and treatment can produce hyperchloraemic normal-gap acidosis. For that reason, the consensus does not recommend the anion gap as a first-line diagnostic or resolution criterion. Do not use gap normalisation by itself to declare DKA resolved.
A normal gap does not rule out elevated lactate
A normal anion gap does not exclude hyperlactataemia. A systematic review and diagnostic-accuracy meta-analysis found low sensitivity for detecting moderate hyperlactataemia, so the anion gap should not be used as a substitute screening test for lactate. Measure lactate directly when clinically indicated.
If the clinical question also involves a measured osmolality, calculate the serum osmolality / osmol gap → separately, then use the Osmolality and Osmol Gap Guide → if you need the interpretation. The osmol gap and anion gap are complementary calculations, not interchangeable results.
Normal anion gap metabolic acidosis (NAGMA)
Metabolic acidosis can occur while the anion gap remains within the local reference interval. In this pattern, bicarbonate falls while chloride rises, so the calculated gap may stay relatively unchanged. Common examples include gastrointestinal bicarbonate loss, renal tubular acidosis and chloride-rich fluid administration.
Low or negative anion gap
A low or negative gap is not automatically a calculator error, but testing or measurement error is the most common causedescribed in the 2023 Cleveland Clinic practical review. Recheck the entered sodium, chloride and bicarbonate and, when clinically appropriate, confirm the result against repeat or original laboratory data before building a differential around it.
Other recognised explanations include hypoalbuminaemia, an increase in unmeasured cations such as lithium, positively charged monoclonal proteins, and some analytical effects that overestimate chloride or underestimate sodium. A persistently low or negative gap therefore needs context rather than a universal calculator label.
Delta gap / delta ratio
When a high-gap metabolic acidosis is already established, clinicians may compare the rise in anion gap with the fall in bicarbonate to look for an additional metabolic process. That is a separate calculation with its own assumptions and is not currently included in the MedMaths anion-gap calculator.
Calculate first, interpret second
The calculator deliberately does not label one numeric result universally “normal” or “high”. Use the reporting laboratory's reference interval, then apply the interpretation framework above. The gap is a clue, not a stand-alone diagnosis.
Sources
Umpierrez GE, et al. Hyperglycemic Crises in Adults With Diabetes: A Consensus Report. Diabetes Care. 2024;47(8):1257–1275. doi:10.2337/dci24-0032. Open-access article.
American Diabetes Association Professional Practice Committee. Diabetes Care in the Hospital: Standards of Care in Diabetes—2026. Diabetes Care. 2026;49(Suppl 1). ADA source.
Haber LA, Dhaliwal G, Lo L, Rizzuto G. Evaluating a low anion gap: A practical approach. Cleve Clin J Med. 2023;90(10):619–623. doi:10.3949/ccjm.90a.23035. Article.
Fenves AZ, Emmett M. Approach to Patients With High Anion Gap Metabolic Acidosis: Core Curriculum 2021. Am J Kidney Dis. 2021;78(4):590–600. PMID 34400023. PubMed.
Mehta AN, Emmett JB, Emmett M. GOLD MARK: an anion gap mnemonic for the 21st century. Lancet. 2008;372(9642):892. PMID 18790311. PubMed.
Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807–1810. PMID 9824071. PubMed.
Posen AK, Paloucek FP, Petzel R. Anion gap physiology and faults of the correction formula. Am J Health Syst Pharm. 2022;79(6):446–451. PMID 34788391. PubMed.
Chionh CYC, et al. Serum anion gap revisited: a verified reference interval for contemporary use. Intern Med J. 2022;52(9):1531–1537. PMID 34028972. PubMed.
Glasmacher SA, Stones W. A systematic review and diagnostic test accuracy meta-analysis of the validity of anion gap as a screening tool for hyperlactatemia. BMC Res Notes. 2017;10:556. doi:10.1186/s13104-017-2853-9. Open-access article.