Chloride Blood Test: What It Measures, What's Normal, and What It Means for Athletes

Key Takeaways: Chloride Blood Test

Chloride is the main negatively charged ion in the fluid outside cells. Plasma normally holds about 97–107 mmol/L, against roughly 90 mmol/L inside red blood cells and only 2–5 mmol/L in skeletal muscle.[1]
No single cut-off applies everywhere: low chloride is usually set somewhere below 96–101 mmol/L and high chloride somewhere above 106–111 mmol/L, depending on the laboratory.[1]
Chloride is best read next to sodium and bicarbonate. When chloride moves together with sodium, in a ratio of about 1:1.4, the pattern fits a water-balance problem; a low chloride with a high bicarbonate, independent of sodium, fits metabolic alkalosis, although compensation for chronic respiratory acidosis can produce the same pattern.[2]
Among 51,060 US adults, the group with the lowest chloride (101.2 mmol/L or less) had the highest mortality over a median follow-up of 111 months. That cut-off was a statistical quartile boundary, not a clinical threshold, and the study was observational and measured chloride only once.[3]
Blood chloride can rise or fall around exercise, depending on the type of effort and the circumstances. In small studies, it rose from 101.8 to 107.9 mmol/L in arterial plasma right after a 2-minute cycling bout to exhaustion,[4] showed no significant change across a cool-weather marathon,[5] and fell significantly after a 160-km trail race.[6]
In trained cyclists, blood chloride drifted down during the hours after a 0.3 g/kg dose of sodium bicarbonate taken at rest.[7]
In my clinical experience, chloride is seldom measured or acted on in primary care. It matters mainly in acute situations where a blood gas analysis is used to assess acid–base balance, such as diabetic ketoacidosis, sepsis, trauma or hypovolaemic shock.
In my view, being an athlete is rarely a reason in itself to test chloride. In my practice, testing follows the clinical situation, for example exertional rhabdomyolysis, and in someone without symptoms it is seldom needed.
In my view, intravenous sodium chloride is not quite as harmless as it may seem, because large volumes can push bicarbonate down and contribute to acidosis. When I decide on intravenous fluids in hospital, I usually choose Ringer’s acetate, although the right fluid depends on the patient.

Introduction: Chloride Blood Test

Chloride is rarely at the centre of medicine in the way sodium and potassium are. Its concentration does matter in some clinical situations, intravenous fluid correction of dehydration in children being one example. In my clinical experience of adult medicine, at least in primary care, it is seldom even measured and very rarely leads to any clinical conclusions, and the routine laboratory panels I see often do not include it at all. In my own clinical work in Finland, I most often come across chloride in blood gas analyses. Chloride is not meaningless, but in my view it has very little place in everyday medicine, which is why it is in many ways a somewhat forgotten electrolyte.

Where chloride is measured, for example on a basic metabolic panel alongside sodium, potassium and bicarbonate, it is easy to read past. Yet when researchers followed 51,060 adults from the US National Health and Nutrition Examination Survey for a median of 111 months, the people in the lowest chloride quartile, at 101.2 mmol/L or below, consistently had the highest risk of death.[3] The top of that quartile still lies inside the 96–106 mmol/L range that the same authors describe as the widely accepted normal range for adults.[3]

That does not make chloride a hidden danger sign, but when it is measured, the number is worth reading in context. It can reflect two things at once: how much water the body holds relative to salt, and how its acid–base balance is set.[2] For athletes, there is a third layer. A maximal effort, a long race, sweat and fluid intake, a bicarbonate supplement or a weight cut may all move the result.

Below: what the chloride blood test measures, what counts as normal, what high and low values usually mean, and how training changes the picture. For the physiology of chloride in sport in more depth, see Chloride in Athletes.

What Does a Chloride Blood Test Measure?

A chloride blood test measures the concentration of chloride ions in serum or plasma. Chloride is the principal anion of the extracellular fluid, and the body depends on it to keep electrical charges balanced and to regulate acid–base status.[1] It also takes part in blood pressure control, carbon dioxide transport, kidney function and the production of hydrochloric acid in the stomach.[1]

The concentration differs sharply between body compartments. Plasma carries about 97–107 mmol/L, red blood cells about 90 mmol/L and skeletal muscle cells only 2–5 mmol/L.[1] The blood test captures only the chloride in serum or plasma, not the chloride inside cells.

Chloride and bicarbonate are inversely related: when one rises, the other often falls.[1] This is why the result is hard to interpret on its own. A review of metabolic alkalosis explains that a high or low chloride may point to a problem with body water, with acid–base balance, or with both at once.[2] When water balance is the problem, chloride and sodium shift in proportion, at about 1:1.4.[2] When the problem is metabolic alkalosis, bicarbonate climbs and chloride falls reciprocally, without a matching change in sodium.[2] The same electrolyte pattern can also come from compensation for chronic respiratory acidosis, so blood pH and the clinical picture are needed to tell them apart.[2]

Chloride is also one of the values used, together with sodium and bicarbonate, to calculate the anion gap.

From a clinical perspective, in my experience chloride matters mostly where acid–base balance changes, although research has also linked it to long-term prognosis, as discussed below. Chloride and bicarbonate work together to maintain electroneutrality, and in many conditions the body compensates by adjusting the amount of bicarbonate. In my observation, acid–base disorders are less common in primary care, at least in a non-urgent elective clinic, so its clinical significance there is limited to a few situations. In my experience, these situations are usually acute by nature and need emergency care in one way or another.

In my clinical experience, chloride becomes clinically significant in many acute situations where a blood gas analysis is needed to assess acidosis or alkalosis. These patients include many surgical patients, patients with ischaemia, poisonings, diabetic ketoacidosis and people with diabetes more generally, respiratory acidosis, sepsis and trauma, as well as dehydrated patients and those in hypovolaemic shock. In these situations, chloride is typically measured as part of the blood gas analysis. Dialysis patients are a separate case. Their electrolyte and acid–base monitoring is part of specialist nephrology care, and in my experience a general practitioner rarely deals with their chloride directly.

An ordinary clinic visit can also turn into an acute case, for example when a patient turns out to have diabetes. Recently, I saw a patient in whom a high blood glucose was an incidental finding and diabetic ketoacidosis was suspected. In my view, it makes sense to check blood chloride alongside the other electrolytes in that situation, because ketoacidosis can disturb acid–base balance. Getting the fluid therapy right matters too. In my experience, it follows a precise algorithm, but that belongs largely to the expertise of internal medicine and the emergency physician, and it is carried out in hospital.

The blood test is not the sweat chloride test

The sweat chloride test answers a different question. It measures chloride in sweat and is used to diagnose cystic fibrosis. Consensus guidelines written by 32 experts from 9 countries state that cystic fibrosis can be diagnosed when sweat chloride reaches 60 mmol/L or more in someone with a positive newborn screen, compatible clinical features or a positive family history.[8] In screen-positive infants, a value below 30 mmol/L makes the disease unlikely, while values of 30–59 mmol/L on two separate occasions fall into an intermediate range that calls for further testing.[8] A normal chloride blood test is not a substitute for the sweat test.

Chloride Blood Test Normal Range

There is no universal reference interval. A review of chloride in critical illness notes that low chloride is usually defined as below 96–101 mmol/L and high chloride as above 106–111 mmol/L, with the exact limits set by the local laboratory.[1] The NHANES authors cite 96–106 mmol/L as the widely acknowledged adult range; in their own population, measured with indirect ion-selective electrode methods, the median was 103.2 mmol/L, with an interquartile range of 101.2–105.0 mmol/L.[3]

Chloride is tightly linked to body water and reacts readily to plasma concentration or dilution.[1] A chloride blood test result a few units outside the printed range therefore needs context: what sodium and bicarbonate show, whether there has been vomiting, diarrhoea or diuretic use, and what happened in the hours before the sample. Compare your value with your own laboratory’s reference interval, not a number from the internet.

What Causes Low Chloride?

In hospital and intensive care data, low chloride, or hypochloremia, has four broad mechanisms.

Losses from the gut. Vomiting and diarrhoea both remove chloride.[1] Stomach fluid contains about 150 meq/L of chloride.[2] When it is vomited, the acid never reaches the small intestine to be neutralised, so bicarbonate is left behind in the body, and metabolic alkalosis with low chloride can follow.[2]

Losses through the kidneys. Chloride can be lost when the kidneys fail to reabsorb it adequately, or when they hold on to extra bicarbonate, as happens in chronic respiratory acidosis or hyperaldosteronism.[1] Thiazide and loop diuretics commonly cause low potassium and metabolic alkalosis,[2] and furosemide in particular is clearly associated with metabolic alkalosis.[1]

From a clinical perspective, diuretic use is one situation where chloride can become relevant, because some diuretics can change chloride concentrations in ways that may cause problems. Occasionally, a patient on diuretics or with kidney failure may have developed an electrolyte disorder slowly, without it necessarily causing symptoms, and in my clinical experience it can even come to light on a screening test. In my experience, a clinically significant disturbance of this kind may need urgent assessment, depending on its severity and the patient’s overall condition.

Dilution. In hospital patients, infusion of hypotonic fluids can dilute chloride.[1] When the cause is a water problem, sodium falls alongside chloride,[2] which links low chloride to low sodium in athletes.

A bicarbonate load. When large volumes of bicarbonate are infused, chloride may be exchanged for bicarbonate to keep the charges balanced.[1]

When low chloride comes with metabolic alkalosis, a spot urine chloride helps a doctor sort the cause. A urine chloride below 20 meq/L (for chloride, the same as mmol/L) fits a reduced fluid volume, as after vomiting or once a diuretic has worn off; a value above 20 meq/L can occur either with an expanded volume or with ongoing chloride loss through the kidneys, as with an actively working diuretic or a kidney tubule reabsorption defect such as Bartter or Gitelman syndrome.[2]

What Causes High Chloride?

In critically ill patients, high chloride, or hyperchloremia, comes mainly from three mechanisms: loss of bicarbonate through the gut or kidneys, dilution with fluids that contain little bicarbonate, or a surplus of chloride-rich fluids.[1] Diarrhoea is probably the most frequent cause of bicarbonate loss in intensive care, and some forms of renal tubular acidosis cause bicarbonate loss through the kidneys.[1] In critical illness, chloride has been identified as the main factor behind non-anion gap metabolic acidosis,[1] the pattern explained in the anion gap article. Note that the same review lists diarrhoea among the causes of both low and high chloride.

In critical illness, chloride-rich infusions such as normal saline are the most frequent cause.[1] A 0.9% sodium chloride solution contains 154 mmol/L of chloride, and even healthy volunteers can develop hyperchloremic metabolic acidosis after 2,000 mL of chloride-rich intravenous fluid in total.[1] In intensive care, hyperchloremia has been observed in about 25–45% of patients.[1] Acetazolamide, triamterene and certain corticosteroid derivatives that cause salt retention are also listed as causes.[1]

From a clinical perspective, this is one situation where chloride can become relevant. If a patient in hospital receives intravenous so-called physiological saline, the extra chloride can push bicarbonate down, which can in turn tip the patient into acidosis. In that sense, an intravenous sodium chloride drip is not quite as harmless as it is often thought to be. In my clinical experience, this is one reason why the hospitals I have worked in often use Ringer’s acetate instead, and I typically do the same: when I decide on intravenous fluids in hospital, I usually choose Ringer’s acetate rather than sodium chloride, and I rarely give sodium chloride for long. In my view, one advantage of Ringer’s acetate is its lower chloride load and acetate buffer, which may reduce the risk of hyperchloremic metabolic acidosis compared with large volumes of normal saline. The right fluid still depends on the patient’s situation.

A high chloride can also reflect a water deficit; in that case, sodium rises with it in roughly the usual 1:1.4 proportion.[2]

Low Chloride and Long-Term Health

A large population analysis comes from NHANES. It covered 10 survey cycles from 1999 to 2018 and recorded 7,582 deaths among 51,060 adults.[3] Compared with the lowest quartile (101.2 mmol/L or less), the adjusted hazard ratios for all-cause mortality in the three higher quartiles were 0.77, 0.72 and 0.77.[3] The relationship was L-shaped: risk rose mainly at the low end, and the highest quartile (105.1 mmol/L or more) had lower, not higher, risk than the lowest.[3] Overall, lower chloride was also associated with higher cardiovascular, cancer and respiratory mortality, although the authors note that an earlier NHANES cohort of 16,483 people did not find a link between low chloride and cardiovascular death.[3] The authors list important limits: the design cannot establish causation, chloride was measured only once, the participants were US civilians, and residual confounding cannot be excluded.[3]

Heart failure research points the same way. In 1,318 patients admitted with acute decompensated heart failure, each 1-unit higher admission chloride was associated with lower long-term mortality (hazard ratio 0.94 per unit); after adjustment, chloride stayed significant (0.93 per unit) while sodium did not.[9] Results were similar for 1-year mortality in an independent cohort of 876 patients.[9] A meta-analysis of 15 studies and 25,848 heart failure patients reported a hypochloremia prevalence of 8.6–31.5%; having hypochloremia carried a hazard ratio for all-cause mortality of 1.56, and each 1 mmol/L lower chloride was associated with a hazard ratio of 1.06.[10]

These studies treat the chloride blood test as a risk marker, not a target. None of them tested whether raising chloride improves survival.

The Chloride Blood Test in Athletes

In my view, being an athlete is rarely a reason in itself to measure chloride. In my practice, electrolyte testing in athletes generally follows the clinical situation. When an athlete has an actual medical condition, it is natural to include chloride with a blood gas analysis; exertional rhabdomyolysis is one example where it would become relevant. Otherwise, as I see it, athletes do not usually need to be screened for chloride unless their clinical condition calls for it, and in someone without symptoms the test is seldom needed.

Training may affect the chloride blood test through fluid shifts, chloride moving into red blood cells, sweat and drinking, and supplements. The studies below used different designs, samples and timings, so their numbers are not directly comparable.

After a short, exhausting effort

In a Norwegian laboratory study, seven young, healthy male volunteers rode a cycle ergometer at a fixed workload until they could not continue, which took 2 minutes; blood came from catheters placed in the femoral artery and vein.[4] In samples taken 20–30 seconds after the effort, arterial plasma chloride had risen from 101.8 to 107.9 mmol/L.[4] Sodium increased by about 10 mmol/L, mainly because plasma became more concentrated, and chloride rose somewhat less than sodium.[4] At the same time, the calculated chloride concentration inside red blood cells went up, reflecting the chloride shift.[4] Red blood cells let chloride cross their membrane almost freely and swap it for bicarbonate, an exchange that appears key to how blood carries carbon dioxide.[1] By the 1-hour mark, plasma electrolyte levels had returned to where they started, with lactate and bicarbonate the exceptions, although the authors note that flushing the catheters with isotonic saline makes later recovery changes difficult to interpret.[4]

Those were catheter samples taken within a minute of a single exhausting bout, not routine clinic draws. Only the arterial value went above 106 mmol/L; femoral-venous plasma rose from 100.5 to 105.3 mmol/L.[4] Still, the result shows that chloride can rise toward or above the upper limit for a short time after a maximal effort.

During a cool-weather marathon

At the 1987 Pittsburgh Marathon, 45 runners (39 men, 6 women) were tested before and after a race run in rain at 46°F (about 8°C).[5] They drank 1,650 mL on average and finished in a mean time of 4 hours 1 minute.[5] Serum sodium rose significantly, but serum chloride did not change significantly, and the average weight loss was 1.9 kg.[5]

After a 160-km ultramarathon

In a 160-km competitive trail race, 54 runners had blood drawn the day before the race and immediately after it; 29 took ibuprofen and 25 did not.[6] Body weight did not change significantly during the race, and both groups finished in about 25.6–25.8 hours.[6] The abstract reports that serum sodium, potassium and chloride were all significantly lower after the race, with no significant effect of ibuprofen on these markers.[6] The published abstract gives significance levels, not the chloride values.[6]

Chloride falling together with sodium is compatible with a water-balance contribution, as described above.[2] Applying that principle to this race is my interpretation; the study did not determine the mechanism or exclude acid–base effects.

Sweat and chloride

Sweat starts out nearly isotonic with plasma, but the sweat duct reabsorbs sodium and chloride, so the sweat that reaches the skin is hypotonic.[11] Among the sweat constituents the review summarises, sodium and chloride are the most concentrated, at roughly 10 to 90 mmol/L.[11] People whose sweat is salty, around 70–80 mmol/L or more, are more prone to sodium chloride imbalance when they sweat heavily for long periods.[11] Even so, the salt content of sweat does not predict how hydrated someone is or how fast they sweat.[11] Both authors of this review are affiliated with the Gatorade Sports Science Institute (PepsiCo).[11]

Because final sweat is hypotonic, sweat loss on its own removes proportionally more water than chloride. That is my inference from the physiology, not a finding of the studies above, and it suggests that what an athlete drinks, not only what they sweat, shapes the blood value after a long event. Cystic fibrosis is the exception that shows the link: its sweat is abnormally salty,[11] and major sweating in cystic fibrosis is listed among the causes of metabolic alkalosis with low urine chloride.[2] Sodium follows a similar logic, covered in Sodium in Athletes.

Sodium bicarbonate supplements

Some athletes take sodium bicarbonate to increase the blood’s buffering capacity.[7] In a randomised crossover study, 12 well-trained male cyclists took 0.3 g/kg of sodium bicarbonate in two different formats and then rested, with capillary blood sampled every 30 minutes for 300 minutes.[7] Blood chloride fell over time after ingestion, with no statistically significant difference between the formats.[7] Chloride returned to a level not significantly different from baseline with capsules, but stayed lower with the hydrogel mini-tablet system.[7] The study had no placebo arm,[7] so the fall is linked to bicarbonate in time rather than proven to be caused by it. The authors had received university funding from the manufacturer of the hydrogel product, with a contractual right to publish regardless of the results.[7]

This fits the reciprocal relationship between bicarbonate and chloride,[2] and a similar exchange has been described with high-volume intravenous bicarbonate in hospital.[1] My practical inference: a chloride blood test taken within hours of bicarbonate loading may read lower for this reason, so tell the doctor about the supplement.

Weight cutting, vomiting and diuretics

Rapid weight loss before a weigh-in is covered in Weight Cutting in Combat Sports. If a weight cut involves vomiting or diuretics, my inference is that the chloride blood test may show it, depending on timing and severity; no cited study looked at weight-cutting athletes. Vomiting removes chloride-rich stomach acid and can produce metabolic alkalosis with low chloride.[2] Loop and thiazide diuretics do the same through the kidneys.[2][1]

Urine chloride can help narrow down the cause, although a single result is not definitive. In metabolic alkalosis, it is typically below 20 meq/L after vomiting, high while a diuretic is acting, and low again once the effect wears off.[2] The review notes that urine chloride swinging widely between tests suggests diuretic use, which some patients may not disclose.[2]

Conclusion: Chloride Blood Test

A chloride blood test is a small number with a broad reach. Plasma chloride normally sits around 97–107 mmol/L,[1] and it shifts with water balance and acid–base status.[2] In 51,060 US adults, the lowest chloride quartile (101.2 mmol/L or below, a statistical grouping rather than a clinical threshold) had the highest observed mortality,[3] and in heart failure each 1 mmol/L lower chloride was associated with a hazard ratio of 1.06.[10]

For athletes, context decides. Arterial chloride rose to 107.9 mmol/L right after 2 minutes of cycling to exhaustion,[4] did not change significantly in a cool marathon,[5] fell after a 160-km race,[6] and declined after bicarbonate loading.[7] Read the result with sodium and bicarbonate, note what you did in the hours before the sample, and if something does not add up, discuss with your doctor whether a repeat test is needed.

In my experience, chloride is seldom at the centre of clinical decisions, at least in primary care. Its clinical weight lies mainly in acute acid–base disturbances, where it is measured with a blood gas analysis, and in situations such as intravenous saline or diuretic use. In my view, an athlete without symptoms rarely needs a chloride test on its own; the clinical situation decides.