Math
Osmolality and water-deficit formulas: a fictional spreadsheet exercise
Practice units, arithmetic and input checks with fictional laboratory values. Results are educational calculations, not diagnoses or fluid-treatment instructions.
Keep formula conventions and units explicit
Use only the fictional values provided to explore spreadsheet arithmetic. The example estimates osmolality as 2×sodium + glucose/18 + BUN/2.8. Sodium is in mEq/L; glucose and blood urea nitrogen are in mg/dL. BUN is not the same mass measurement as total urea. Dividing by these factors converts the glucose and urea-related terms to the corresponding mmol/L quantities.
Measured minus calculated osmolality is a signed gap. Laboratories use different calculation formulas and reference intervals; for example the University of Iowa includes an additional glucose factor. A gap by itself neither diagnoses nor excludes toxic alcohol exposure, and this exercise deliberately produces no Normal or Elevated classification.
The separate water-deficit arithmetic uses a supplied fictional total-body-water estimate and an illustrative sodium target of 140. It excludes ongoing losses and other clinical factors. The result is not an amount to drink or administer, a correction rate, or a treatment plan; those require clinical assessment.
| Lab Analyte / Metric | Cell | Fictional input / arithmetic | Educational result |
|---|---|---|---|
| Sodium, Glucose, BUN | B2=140, B3=90, B4=14 | 2*140 + 90/18 + 14/2.8 | 280 + 5 + 5 = 290.0 mOsm/kg |
| Measured Osmolality | B5 = 308 mOsm/kg | 308 - 290.0 | Signed gap =18.0; no diagnostic label |
| Total Body Water (TBW) | B6=42 L, supplied fictional TBW estimate | At Na = 140 -> 42*(1-1) | 0.00 L Free Water Deficit |
| Alternative fictional sodium 154 | B2 set to 154 mEq/L | 42 * (154 / 140 - 1) | Model result 4.20 L; not a treatment volume |
Calculated Serum Osmolality (mOsm/kg) from US conventional units
This version uses glucose and BUN in mg/dL. Do not paste glucose or urea values in mmol/L into the same inputs; that would apply the conversions twice. Use a separately specified formula for different units.
=IF(OR(COUNT(B2:B4)<>3,B2<=0,MIN(B3:B4)<0),"Invalid inputs",ROUND(2 * B2 + (B3 / 18) + (B4 / 2.8), 1))Signed measured-minus-calculated gap
The gap is reported numerically without diagnostic interpretation. The optional water-deficit exercise below reports Not applicable below the illustrative target instead of implying that a zero value establishes normal water balance.
=IF(OR(COUNT(B2:B5)<>4,B2<=0,MIN(B3:B5)<0),"Invalid inputs",ROUND(B5 - (2 * B2 + (B3 / 18) + (B4 / 2.8)), 1))Optional fictional water-deficit calculation
TBW is supplied in liters; the target 140 is illustrative. At the target the arithmetic result is zero; below it this example returns Not applicable. Do not use the result to choose fluid replacement.
=IF(OR(COUNT(B2)<>1,COUNT(B6)<>1,B2<=0,B6<=0),"Invalid inputs",IF(B2<140,"Not applicable",ROUND(B6*(B2/140-1),2)))Try the example
Use the original values or change them to test your understanding.