Anion Gap (AGAP) Blood Test: Low, High, and What Counts as Normal
Other names: Anion Gap, AGAP, ANION GAP, A GAP, ANGAP, Anion Gap Ser/Plas, Anion Gap SerPl, Anion Gap Serum/Plasma, Anion Gap w/o K, Anion Gap with K, Anion Gap POC, Anion Gap-TL, Calculated Anion Gap, poc anion gap
Optimal Result:
8 - 16 mEq/L.
At a Glance
The anion gap is not measured. It is calculated from electrolytes that are — sodium, chloride and bicarbonate, and in one of the two formulas potassium as well. Mayo Clinic Laboratories states the version it uses plainly: "A gap = Na - (Cl + HCO3[-])". The gap is what is left over, and it stands in for the charged particles the panel does not measure directly.
That matters more than it sounds. The Cleveland Clinic Journal of Medicine puts it directly: the gap is "a derived number", and "a preanalytical or analytical error in any of these can lead to a falsely low anion gap."
The range shown on this page is 8 - 16 mEq/L, and published intervals differ from it substantially. Those two numbers have a history: the Cleveland Clinic Journal of Medicine records that "until the 1980s, the reference range for the anion gap was between 8 and 16 mEq/L." Where this page's band came from is not something this page can tell you, and current published intervals differ from it materially — mostly by sitting lower, though not uniformly.
| Source | Interval | Notes |
|---|---|---|
| The range stored on this page | 8 - 16 mEq/L | Numerically the same band CCJM describes as historically used |
| Mayo Clinic Laboratories (electrolyte panel) | 7 - 15, age 7 and over | Not established below age 7 |
| StatPearls | 8 - 12 without potassium; 12 - 16 with potassium | Two ranges for two formulas |
| Sadjadi and colleagues, 2013 | 3 - 11 measured | In 409 people with normal kidney function and albumin |
| Cleveland Clinic Journal of Medicine, 2023 | 3 - 9 mEq/L | Defines low as 3 mEq/L or less |
| ARUP Laboratories | No number published | "By report (reports may vary based on instrumentation)" |
There is a documented reason the numbers fell. Sadjadi and colleagues explain it: "Using flame photometry technique in the 1970s, the normal value of anion gap (AG) was determined to be 12 ± 4 meq/L", and "Compared with flame photometry, the ion-selective electrode (ISE) tends to give a higher concentration of serum chloride, so the AG value has fallen over time." The chemistry in the patient did not change; the instrument did.
So a number that looks low here may be ordinary on a modern analyzer. In that study, 409 people with an eGFR of 60 or above and albumin of 4 g/dL or more had a mean gap of 7.2 and a range of 3 to 11 — and the authors propose that "a value <2 meq/L should be considered a low anion gap". A result of 5 or 6 sits inside their range and below the band stored here. Read your number against the interval printed on your own report.
Two formulas are in use, and their published ranges differ by about four units. StatPearls gives both — "anion gap = (Na + K) – (Cl + HCO3)" and "anion gap = (Na) – (Cl + HCO3)" — and the paired ranges: "A normal anion gap is 8 to 12 (if potassium is included, normal values are 12 to 16)." A result labeled w/o K was calculated the second way, and needs a range written for that version.
Albumin pulls the gap down, and the reported number is not corrected for it. Albumin is a negatively charged protein and part of what the gap represents. Figge and colleagues measured the effect: "Each g/L decrease in serum albumin caused the observed anion gap to underestimate the total concentration of gap anions by 0.25 mEq/L". The Cleveland Clinic Journal of Medicine describes one published correction, which "adjusts a patient's anion gap by adding 2.5 mEq/L to the calculated anion gap for each decrease of 1 g/dL in albumin from a normal baseline of 4 g/dL", because "hypoalbuminemia may conceal an elevated anion gap metabolic acidosis."
A low gap is often a measurement problem, not a disease. The same paper is unambiguous: "Measurement error in serum chemistry is the most common cause of a low anion gap", and its first step is a repeat sample "to exclude testing process error". MedlinePlus describes the same practice: a low result "is so rare that your provider will usually have you tested again to make sure the results are accurate."
How to read your result
These rows are written against the 8 - 16 mEq/L band stored on this page, and name where the published intervals disagree.
| Your result | What it means | What happens next |
|---|---|---|
| Below 8 mEq/L | Flagged low here. Between 4 and 7 the stored band and the published intervals disagree most — those values sit below this page's floor and inside the 3 to 11 Sadjadi and colleagues measured in people with normal kidney function and normal albumin. Lower than that, see the row beneath. | Check the range printed on your own report first. If it is low there too, a repeat sample is the standard next step, and albumin is what to look at if it stays low. |
| 3 mEq/L or below | The sources draw the low line in three places. This page flags below 8; the Cleveland Clinic Journal of Medicine defines low as "less than or equal to 3 mEq/L"; Sadjadi and colleagues say "a value <2 meq/L should be considered a low anion gap". So 3 is low here and by that journal's definition, and still inside the 3 to 11 Sadjadi measured. | Measurement error is the most common explanation and a repeat is the first step. If it persists, that paper says a low gap "should prompt investigation for a monoclonal gammopathy". |
| 8 to 16 mEq/L | Inside the band shown here — but not inside every published interval: 10 and above sits over the Cleveland Clinic Journal of Medicine's 3 - 9, 12 and above over the 3 to 11 Sadjadi measured in people with normal kidney function and normal albumin, and 16 over Mayo's ceiling of 15 for ages 7 and over. | Read against your own report's range. If your laboratory prints a lower ceiling than 16, your result may be flagged there and not here. |
| Above 16 up to 18 mEq/L | Above the band shown here, and above every interval in the table above. | Worth raising, with your albumin and your bicarbonate, since both change how the number reads. |
| Above 18 mEq/L | StatPearls states that "an anion gap >18 almost always indicates the presence of an organic acid" — for the calculation without potassium. No source here gives an equivalent figure for the version that includes potassium, so it does not transfer. | Take it to the clinician who ordered the test, with the rest of the panel. |
| Any result, if albumin is low | The measured gap understates the real one. Add 2.5 for every 1 g/dL your albumin sits below 4. | Ask whether the corrected gap changes the picture. |
| Any result labeled w/o K | Calculated without potassium. StatPearls' published range for that version is 8 - 12, against 12 - 16 for the version that includes it. | Check that the range you compare against was written for the same formula. |
If you remember only one thing
The anion gap is a calculated number, and what counts as normal for it depends on the analyzer that produced it. Published intervals run from 3 - 9 to 8 - 16 above, and they fell because instruments changed, not because people did. Read your result against the interval printed on your own report, and if it is flagged low, know that measurement error is the most common cause.
Two people, same result
Two people both have an anion gap of 16 mEq/L.
The first is reading it here, against 8 - 16 — the top of the band, so it shows in range.
The second had the sample run by a laboratory that prints a lower ceiling — Mayo Clinic Laboratories publishes 7 to 15 for people aged 7 and over. There, 16 is above the interval.
Same number, two verdicts, separated by nothing but which range it is held against. The disagreement lives at the edges of the band, which is exactly where a reader goes looking for an answer.
What this result cannot tell you
Whether you have metabolic acidosis. The gap is one input and bicarbonate another; neither settles it. StatPearls needs the pH: "a low pH with low bicarbonate (HCO3) suggests metabolic acidosis, while a low pH with elevated pCO2 indicates respiratory acidosis."
Whether your gap is truly normal, if your albumin is low. Without the albumin the correction cannot be applied, and a raised gap can be hidden. Your albumin may not be on the same panel.
Common interpretation mistakes
Comparing your number to a range from somewhere else. ARUP publishes none, stating that reports "may vary based on instrumentation".
Reading a low gap as a diagnosis. Measurement error in one of the inputs is why a repeat comes before a search for a cause.
Turning a low gap into "cancer". The link is narrow: plasma cell dyscrasias such as multiple myeloma can produce positively charged immunoglobulin, which lowers the gap. That is a reason to check for a monoclonal protein, not a statement about cancer in general.
Assuming a mildly raised gap means the same as a markedly raised one. The only threshold any source here states is StatPearls' ">18 almost always indicates the presence of an organic acid", and that belongs to the calculation without potassium.
Questions your doctor may ask
- What was your albumin on the same sample?
- What were your bicarbonate, chloride and sodium?
- Have you had diarrhea, vomiting, or been dehydrated recently?
- Do you have diabetes or kidney disease?
- What medicines are you taking, including aspirin and lithium?
- Has your gap been low or high before?
Read together with
- Carbon Dioxide (CO2) — the bicarbonate term, and central to the acid-base picture
- Chloride — the input whose measurement method is why the ranges fell
- Sodium — the positive term the gap is measured from
- Albumin — decides whether a normal-looking gap is really normal
- Potassium — in one of the two formulas, not the other
- Creatinine — kidney function, read alongside a raised gap
- Glucose — read alongside a raised gap in diabetes
Clinical pearls
Sadjadi and colleagues go further than describing the fall: "We believe the value of the AG needs to be lowered to 6-7 meq/L and that every laboratory should have an established value for AG."
A persistently low gap with a normal albumin earns a second look, because a testing error and a low albumin have both been ruled out.
Clinical Takeaway
The serum anion gap is a calculated quantity, sodium minus chloride and bicarbonate, with a second form that adds potassium and whose published range is four units higher. Its reference interval is analyzer-dependent and has fallen with the move from flame photometry to ion-selective electrodes, which read chloride higher; published intervals in current sources range from 3 - 9 to 8 - 16 mEq/L, and ARUP declines to publish one at all. The band stored on this page is 8 - 16 mEq/L — numerically the interval the Cleveland Clinic Journal of Medicine records as in use until the 1980s, though its provenance here is not established — so results between 4 and 8 are flagged low here while sitting inside the range measured in people with normal kidney function and normal albumin. The low threshold itself is unsettled: this page flags below 8, that journal defines low as 3 or below, and Sadjadi and colleagues propose below 2. Hypoalbuminemia lowers the measured gap by about 2.5 mEq/L per 1 g/dL of albumin below 4 and can conceal a raised gap. A low gap warrants a repeat before anything else, with monoclonal gammopathy the condition to exclude if it persists with a normal albumin. For the calculation that excludes potassium, StatPearls states that above 18 an organic acid is almost always present.
In one sentence
The anion gap is a calculation rather than a measurement, so read it against the interval printed on your own report and alongside your albumin.
Bottom line
Read your anion gap against the range printed on your own report, not against a number from somewhere else. Published intervals differ more than most reference ranges do — from 3 - 9 to 8 - 16 mEq/L among the sources above — because instruments changed the measurement of chloride and the gap fell with it. The band on this page sits at the top of that spread, which is why a result of 5 or 6 is flagged low here while inside the 3 to 11 Sadjadi and colleagues measured in people with normal kidney function and normal albumin.
If your gap is flagged low, the first question is whether it is real. Measurement error in one of the inputs is a commonly cited cause, and a repeat sample is the standard next step. If it stays low and your albumin is normal, that is the version worth following up.
If your gap is raised, your albumin and your bicarbonate change what it means. As MedlinePlus puts it about this test, "normal results may vary by lab."
FAQ about Anion Gap
-
What is a normal anion gap?
There is no single number, and the disagreement between sources is larger than for most reference ranges. This page shows 8 to 16 mEq/L. Mayo Clinic Laboratories publishes 7 to 15 for people aged 7 and over on its electrolyte panel. StatPearls gives 8 to 12 for the formula without potassium and 12 to 16 for the one with it. Sadjadi and colleagues measured a mean of 7.2 with a range of 3 to 11 in 409 people with normal kidney function and albumin, and the Cleveland Clinic Journal of Medicine works to 3 to 9. ARUP Laboratories publishes no number, stating that the interval is "by report (reports may vary based on instrumentation)." Read your result against the interval printed on your own report. -
Why do normal anion gap ranges differ so much between labs?
Because the way chloride is measured changed, and the gap is calculated from chloride. Sadjadi and colleagues describe it: "Using flame photometry technique in the 1970s, the normal value of anion gap (AG) was determined to be 12 ± 4 meq/L", and "Compared with flame photometry, the ion-selective electrode (ISE) tends to give a higher concentration of serum chloride, so the AG value has fallen over time." A higher measured chloride leaves a smaller gap. Laboratories adopted the newer method at different times and use different instruments, which is why ARUP states its interval is by report and varies with instrumentation. -
What does AGAP mean on a blood test?
AGAP is the abbreviation many laboratories print for the anion gap. It is not a substance that was measured — it is calculated from sodium, chloride and bicarbonate, which reports usually label carbon dioxide, with potassium included in one of the two formulas in use. Some reports label it anion gap w/o K, meaning potassium was left out of the calculation. -
What does a low anion gap mean?
Most often, that one of the measured electrolytes behind it was slightly off. The Cleveland Clinic Journal of Medicine states that "measurement error in serum chemistry is the most common cause of a low anion gap", and its first step is repeating the sample. It is also worth checking the range printed on your own report, because this page's band starts at 8 while several published intervals start at 3. Where a low result is confirmed, MedlinePlus states that "the most common cause of a low anion gap is a low level of albumin, a protein in the blood." If albumin is normal and the low gap persists, that paper says it "should prompt investigation for a monoclonal gammopathy." -
My anion gap is 5. Should I be worried?
A result of 5 is flagged low against the 8 to 16 band shown on this page, and it sits inside the range Sadjadi and colleagues measured in 409 people with normal kidney function and normal albumin, which was 3 to 11. It is also above the Cleveland Clinic Journal of Medicine's definition of a low anion gap, which is 3 mEq/L or less. So the first thing to check is the range printed on your own report, because the flag here comes from an interval that is higher than the ones those sources use. If your own laboratory also calls it low, a repeat sample and a look at your albumin are the usual next steps. -
My anion gap is 16. Is that high?
It is the top of the 8 to 16 band shown on this page, so it renders in range here. It is above Mayo Clinic Laboratories' 7 to 15 for people aged 7 and over, above the 3 to 11 Sadjadi and colleagues measured in people with normal kidney function and normal albumin, and above the Cleveland Clinic Journal of Medicine's 3 to 9 — a laboratory printing Mayo's ceiling of 15 would flag the same number. This is the clearest example of why the range printed on your own report is what settles it. On magnitude, StatPearls states that "an anion gap >18 almost always indicates the presence of an organic acid", and 16 is below that. But that threshold belongs to the calculation without potassium, and StatPearls gives no equivalent for the version that includes it — so if yours does, the comparison does not apply. What does apply there is that 16 is the top of StatPearls' published range for that formula, 12 to 16. Which is another way of saying the same thing: the number alone does not settle it, and the interval on your own report does. -
My anion gap is 20. What does that mean?
It is above the band shown on this page and above every published interval named here. StatPearls states that "an anion gap >18 almost always indicates the presence of an organic acid", which is the one magnitude threshold the sources on this page give. It belongs to the calculation without potassium. No source here gives an equivalent figure for the version that includes potassium, so if yours does, this threshold does not apply to it. That is a reason to take the result to the clinician who ordered it along with the rest of the panel — particularly your bicarbonate and your albumin, both of which change how a raised gap reads. It is not by itself a diagnosis. -
What is the anion gap formula?
There are two. Mayo Clinic Laboratories states the one that excludes potassium: "A gap = Na - (Cl + HCO3[-])". StatPearls gives both, as "anion gap = (Na + K) – (Cl + HCO3)" and "anion gap = (Na) – (Cl + HCO3)", and notes the normal ranges differ accordingly: "A normal anion gap is 8 to 12 (if potassium is included, normal values are 12 to 16)." A result labeled w/o K used the version without potassium, and needs to be read against a range written for that version. -
What is a corrected anion gap, and do I need one?
It is the anion gap adjusted for a low albumin. Albumin is negatively charged and part of what the gap represents, so when albumin is low the measured gap understates the real one. Figge and colleagues measured the size of the effect: each 1 g/L fall in albumin makes the observed gap understate by 0.25 mEq/L. The Cleveland Clinic Journal of Medicine describes one published correction, which "adjusts a patient's anion gap by adding 2.5 mEq/L to the calculated anion gap for each decrease of 1 g/dL in albumin from a normal baseline of 4 g/dL", and explains why it matters: "hypoalbuminemia may conceal an elevated anion gap metabolic acidosis." StatPearls prints the same correction against a baseline of 4.0 g/dL but calls the normal albumin 4.5 g/dL in the same passage, so treat the corrected figure as an estimate. If your albumin is normal, the correction changes nothing. -
Can a low anion gap mean cancer?
One specific group of conditions, not cancer in general. The Cleveland Clinic Journal of Medicine explains the mechanism: "A rise in positively charged plasma proteins can reduce the anion gap. The most common excess positively charged proteins are monoclonal immunoglobulins or light chains. Plasma cell dyscrasias such as multiple myeloma can produce positively charged immunoglobulin." That is why it says a low anion gap "should prompt investigation for a monoclonal gammopathy". No source on this page links a low anion gap to solid tumors or to cancer generally, and measurement error remains the most common explanation for the finding. -
What causes a high anion gap?
The two lists people find are not the same list, and it is worth knowing which is which. MedlinePlus says a high gap "may mean that you may have acidosis", and lists causes of that acidosis: "Dehydration, Diarrhea, Too much exercise, Kidney diseases, Diabetes or diabetic ketoacidosis, Salicylate poisoning, for example from too much aspirin, Antifreeze poisoning". Not every acidosis raises the gap — diarrhea is the standard example of one that does not, and StatPearls names it "by far the most common" gastrointestinal cause of the normal-anion-gap kind. The causes tied specifically to a raised gap are the ones StatPearls collects as GOLD MARKeT: glycols, oxoproline, lactic acid, D-lactic acid, methanol, aspirin, renal failure or rhabdomyolysis, ketones and toluene. That list is weighted toward poisonings and critical illness, which is worth knowing before reading it against a routine outpatient panel. -
My anion gap is low but my albumin is normal. What now?
That is the version worth following up, because the first of the non-error causes has been ruled out. The Cleveland Clinic Journal of Medicine's sequence is to exclude a testing error first — "if previous tests do not demonstrate a low anion gap, the patient should undergo repeat serum electrolyte sampling to exclude testing process error" — and then, if it persists, to investigate for a monoclonal gammopathy. It also names lithium, and bromide or iodide being read as chloride by the electrode, as causes to consider in the right context. Check the range printed on your own report as well, since this page's floor of 8 is higher than several published ones.
Lab Results Explained and Tracked
What does it mean if your Anion Gap result is too high?
A result flagged high here is above 16 mEq/L, which is the ceiling of the interval stored for this marker — and no source on this page publishes a higher one. Mayo Clinic Laboratories publishes 7 to 15 for people aged 7 and over, the Cleveland Clinic Journal of Medicine works to 3 to 9, StatPearls gives 12 to 16 for the formula that includes potassium, and Sadjadi and colleagues measured 3 to 11 in people with normal kidney function and normal albumin. So unlike a result inside the band, which can be in range here and raised elsewhere, a value above 16 is above every interval named here. Read it against the range printed on your own report all the same, since that is the one the laboratory that ran your sample stands behind.
Two other results change how a raised gap should be read: your albumin and your bicarbonate. Bicarbonate is one of the numbers the gap is calculated from, so it is always alongside it. Albumin may not be — Mayo Clinic Laboratories' electrolyte panel and ARUP's both report the anion gap without it — so it is worth checking whether yours was measured at all.
Your albumin. Albumin is a negatively charged protein and part of what the gap represents, so a low albumin lowers the measured number. Figge and colleagues quantified it: each 1 g/L fall in albumin makes the observed gap understate the real one by 0.25 mEq/L. In the units most reports use, the Cleveland Clinic Journal of Medicine gives the correction as the measured gap plus 2.5 for every 1 g/dL that albumin sits below 4. If your albumin is low, your corrected gap is higher than the number shown, and that is the figure to discuss.
Your bicarbonate. The gap is one half of an acid-base picture; the bicarbonate result is the other. A raised gap alongside a low bicarbonate means something different from a raised gap with a normal one, and it is not a distinction this page can make from the gap alone.
On magnitude, StatPearls states that "an anion gap >18 almost always indicates the presence of an organic acid." That figure belongs to one of the two calculations — it sits in a discussion that takes 12 as the upper limit of normal, which is the scale for the formula without potassium. StatPearls gives no equivalent figure for the version that includes potassium, and one cannot be worked out from the published ranges: those two ranges sit four apart by convention, while any individual result differs between the formulas by that person's own potassium. So if your result includes potassium, 18 is not the line for it, and this page does not know what is. Below 18, the sources here do not put a number on what a mildly raised result means, and neither does this page. On what raises it, one distinction matters more than any list. MedlinePlus says a high anion gap "may mean that you may have acidosis", and then lists what can cause that acidosis: "Dehydration, Diarrhea, Too much exercise, Kidney diseases, Diabetes or diabetic ketoacidosis, Salicylate poisoning, for example from too much aspirin, Antifreeze poisoning". Those are causes of acidosis, and not all of them raise the gap. Diarrhea is the clearest case: StatPearls names it as the commonest gastrointestinal cause of the normal-anion-gap kind — "of the gastrointestinal causes of NAGMA, diarrhea is by far the most common" — so it belongs to the acidosis that leaves the gap where it was. The causes specifically associated with a raised gap are the ones StatPearls collects as GOLD MARKeT: glycols, oxoproline, lactic acid, D-lactic acid, methanol, aspirin, renal failure or rhabdomyolysis, ketones and toluene. That list is weighted toward poisonings and critical illness and reads alarmingly against a routine outpatient panel, which is worth knowing before reading it at home.
The number is not the only thing that matters. A raised gap points toward metabolic acidosis, and MedlinePlus describes what that can look like: it causes "rapid and deep breathing as your body tries to compensate for it", and "confusion or lethargy may also occur", although "most symptoms are caused by the underlying disease or condition that is causing" it. Its instruction is direct: "seek medical help if you have symptoms of any disease that can cause metabolic acidosis." If you feel unwell alongside this result, that is a reason to seek care now rather than to wait for a repeat panel.
What a raised gap does not do is diagnose anything on its own. It is calculated from measured electrolytes, and it is read together with those results and the rest of the panel.
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What does it mean if your Anion Gap result is too low?
A result flagged low here is below 8 mEq/L, and how far below 8 changes what that means. The band used here starts at 8. Sadjadi and colleagues measured a range of 3 to 11 in 409 people with normal kidney function and normal albumin; the Cleveland Clinic Journal of Medicine works to 3 to 9 and defines a low anion gap as "less than or equal to 3 mEq/L".
A result between 4 and 7 is below the floor used on this page and inside the range those sources describe — this is the span where the interval stored for this marker and the published ones disagree most. Lower down, the sources move closer together without meeting. The Cleveland Clinic Journal of Medicine defines a low anion gap as "less than or equal to 3 mEq/L", so 3 is low by that definition as well as by this page's floor of 8. Sadjadi and colleagues draw it lower still — "a value <2 meq/L should be considered a low anion gap and a possible clue to drug intoxication and paraproteinemic disorders" — and 3 was inside the range they measured in people with normal kidney function and albumin. Three sources, three lines. The further below 8 your result sits, the more of them agree with the flag, and the more the steps below matter. In every case, the first thing to do with a low flag here is to look at the range printed on your own report.
The reason the intervals differ is a change in instruments, not in people. Sadjadi and colleagues explain that the anion gap normal value was established in the 1970s using flame photometry, and that ion-selective electrodes "tend to give a higher concentration of serum chloride, so the AG value has fallen over time." The Cleveland Clinic Journal of Medicine records that "until the 1980s, the reference range for the anion gap was between 8 and 16 mEq/L" — the same two numbers this page uses, though where this page's band came from is not established.
Where a low result is confirmed against your own report, the first question is whether it is real. The Cleveland Clinic Journal of Medicine is direct: "Measurement error in serum chemistry is the most common cause of a low anion gap." Because the gap is calculated from measured electrolytes rather than measured itself, an error in any one of them produces a falsely low result, and that paper's first step is a repeat sample to exclude it. MedlinePlus describes the same practice — a low result "is so rare that your provider will usually have you tested again." MedlinePlus also names what a confirmed low gap can indicate: "a low anion gap test result may mean you have alkalosis."
If it persists, albumin is the next thing to look at. MedlinePlus states it plainly: "the most common cause of a low anion gap is a low level of albumin, a protein in the blood." It is also the first of the non-error causes the Cleveland Clinic Journal of Medicine works through. The two sources rank differently, and both are on this page. MedlinePlus names low albumin as the commonest cause; the Cleveland Clinic Journal of Medicine names measurement error. Neither one explains the other away, and this page does not pretend to reconcile them. What both support is the order of the steps below: repeat the test first, then look at albumin. Where the albumin is normal and the low gap is confirmed, that paper says the finding "should prompt investigation for a monoclonal gammopathy" — the plasma cell disorders, such as multiple myeloma, that can produce positively charged immunoglobulin. Other explanations it names are lithium, and bromide or iodide read as chloride by the electrode. Raised calcium, magnesium or potassium can lower the gap too, though that paper notes these are "usually noticed on direct measurement before reaching the point of markedly affecting the anion gap calculation."
None of that is a conclusion from one low number. It is the order in which the possibilities are worked through, and it begins with repeating the test.
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