A worked blood gas after saline: low SID, expected respiratory compensation, chloride-related differentials and a residual-gap check.
Acidosis after chloride-rich fluid
Consider this synthetic adult case after 0.9% saline administration. Sodium is 140 mmol/L, chloride 112.65 mmol/L and lactate 1 mmol/L. Albumin is 40 g/L. The arterial gas and chemistry are from the same sampling time.
The timing makes fluid-associated acidosis a possibility. It does not establish the cause; compare pre-fluid results and review other losses or renal disease.
Metabolic acidosis with expected respiratory compensation
Arterial sample; gas and chemistry from the same sampling time. Respiratory time course: Acute.
Chloride lowers SID
Sodium minus chloride is 27.35 mmol/L. Chloride is high relative to sodium, reducing measured strong ion difference and contributing to acidosis. A higher SID would have an alkalinising direction.
0.9% saline contains equal sodium and chloride concentrations: its SID is zero. Sufficient administration can lower plasma SID. The effect depends on the clinical setting, volume and other simultaneous changes; this calculator does not model a fluid dose.
The simplified sodium–chloride BE estimate is −10.7 mEq/L, the largest measured acidifying contribution in this case. Lactate is not raised.
Causes to consider
Chloride-rich fluids: check the type, volume and timing, including drug-carrier fluids. Compare chloride and SBE before and after administration.
Gastrointestinal bicarbonate loss: ask about diarrhoea, high-output stoma, fistula or drainage losses.
Renal causes: review kidney function, potassium and medicines such as acetazolamide. If renal tubular acidosis is suspected, assess urinary acidification in context.
Persistent or worsening acidosis needs reassessment. A saline history does not exclude hypoperfusion, ketosis, renal anion retention or another concurrent process.
Do not overlook the residual gap
SIDa is 34.3, SIDe 28.6 and SIG 5.8 mEq/L. SIG is above the app’s conventional 0–2 screening range, so this should not be labelled an isolated chloride disorder from these values alone.
This teaching example preserves the model’s illustrative reference SIG while changing chloride. That reference is a mathematical anchor, not a normal-SIG interval. With a real sample, check the method-matched local interval and consider residual anions if the elevation persists.
The potassium-excluded AG is 11.3 mmol/L, within the default 4–13 interval. AG and SIG account for different ions and have different baselines; they need not produce the same classification.