Estimates the negative charge contributed by inorganic phosphate at a specified pH. This premium guide explains physiology, calculation, interpretation, limitations and evidence.
Overview
Estimates the negative charge contributed by inorganic phosphate at a specified pH.
Clinical Significance
A component of effective SID that becomes more important in marked hyperphosphataemia.
When to Use
- Calculate the phosphate component of Stewart analysis.
- Use contemporaneous pH and phosphate.
- Interpret only as one component.
How It Is Calculated
- Phosphate charge = phosphate × (0.309 × pH − 0.469).
- MedicalC validates required values and rounds only the displayed result.
Interpretation
- A component of effective SID that becomes more important in marked hyperphosphataemia.
- Review source measurements, collection quality and clinical trajectory as well as the final value.
Worked Example
Enter clinically plausible values in the displayed units. MedicalC applies: Phosphate charge = phosphate × (0.309 × pH − 0.469). Verify the result against the source data before use.
Patient Considerations
- Consider kidney function, diuretics, intravenous fluids, nutrition, body composition and rapidly changing physiology.
- For timed urine calculations, confirm collection duration, volume and completeness.
- For electrolyte prescriptions, include all oral, enteral, intravenous and medication sources.
Limitations
- Coefficient conventions differ.
- Measurements must be contemporaneous.
- The value is not a treatment target.
Clinical Pearls
- Use paired serum and urine values obtained as close together as possible when the formula requires them.
- A derived renal index is only as reliable as the collection and assays used.
- Check whether treatment occurred between sampling and interpretation.
Common Mistakes
- Mixing µmol/L and mmol/L creatinine values.
- Using spot concentrations as daily excretion rates.
- Ignoring incomplete timed urine collection.
- Treating a maintenance estimate as a replacement or emergency-correction regimen.
- Using a derived acid–base value without checking albumin, lactate and the blood gas.
Evidence Base
Principal source: Figge J, et al. The role of serum proteins in acid-base equilibria. J Lab Clin Med. 1991.
Frequently Asked Questions
What does the Phosphate Weak-Acid Charge calculator do?
Estimates the negative charge contributed by inorganic phosphate at a specified pH.
Can this result establish a diagnosis?
No. It is an adjunct that must be combined with clinical assessment and current guidance.
Why are timing and units important?
Derived results can be misleading when values are non-contemporaneous or use incompatible units.
When should the calculation be repeated?
Repeat when treatment, kidney function or the relevant measured values change and a new result could influence care.
Does this replace a specialist protocol?
No. Direct measurement, laboratory interpretation and specialist protocols take priority.
References
- Figge J, et al. The role of serum proteins in acid-base equilibria. J Lab Clin Med. 1991. — Primary publication, guideline or authoritative specialty source.
Reviewed by: MedicalC Clinical Editorial Team
Last reviewed: July 2026