Calculations and Reference Ranges

Calculations and Reference Ranges

Core Chemistry Calculations

Chemistry calculations are reliable only when measured values use compatible units and calculated results remain distinct from measured results. Keep full precision through the calculation, then round according to the reporting precision specified by the laboratory or question.

Iron capacity and saturation

Total iron-binding capacity (TIBC) can be calculated from serum iron and unsaturated iron-binding capacity (UIBC) or estimated from a transferrin measurement. The transferrin conversion depends on the method. A current Roche-based procedure uses a factor of 1.18 µg/dL TIBC per mg/dL transferrin, while the theoretical binding relationship is approximately 1.40 to 1.41. Use the factor validated for the method.1,2

TIBC (µg/dL) = transferrin (mg/dL) × validated factor

For a procedure that uses the factor 1.18:

TIBC (µg/dL) = transferrin (mg/dL) × 1.18

With the validated factor of 1.18, a transferrin result of 300 mg/dL gives a TIBC of 354 µg/dL. The theoretical factor of 1.41 gives about 423 µg/dL for the same transferrin result. The method’s validated factor determines which calculation applies.

TIBC and serum iron must come from the same specimen and use the same concentration units for these calculations:

UIBC = TIBC − serum iron

transferrin saturation (%) = (serum iron ÷ TIBC) × 100

Worked example. Serum iron is 30 µg/dL and TIBC is 480 µg/dL.

UIBC = 480 − 30 = 450 µg/dL

transferrin saturation = (30 ÷ 480) × 100 = 6.25%, reported as 6.3%

Bilirubin fractionation

Routine bilirubin fractionation measures total bilirubin and a method-defined direct-reacting fraction. In a diazo assay, the direct result includes conjugated bilirubin and may include delta bilirubin, the fraction covalently bound to albumin. The direct method therefore affects the calculated indirect result.3

Use total and direct bilirubin results from the same specimen and in the same units:

calculated indirect bilirubin = total bilirubin − direct bilirubin

Worked example. Total bilirubin is 10.8 mg/dL and direct bilirubin is 2.4 mg/dL.

calculated indirect bilirubin = 10.8 − 2.4 = 8.4 mg/dL

An indirect-predominant result can support increased bilirubin production or impaired conjugation, whereas a direct-predominant result can support hepatobiliary disease. Interpret the fractions with the remaining laboratory and clinical findings.3

Friedewald LDL and non-HDL cholesterol

The Friedewald equation estimates very-low-density lipoprotein cholesterol (VLDL-C) from triglycerides, then subtracts that estimate and high-density lipoprotein cholesterol (HDL-C) from total cholesterol. The VLDL-C divisor depends on the unit system.4

For results in mg/dL:

LDL-C = total cholesterol − HDL-C − (triglycerides ÷ 5)

For results in mmol/L:

LDL-C = total cholesterol − HDL-C − (triglycerides ÷ 2.2)

The Friedewald equation was developed from fasting specimens. Current guidance accepts standard fasting or nonfasting lipid profiles for most patients, but the laboratory’s validated pathway determines which estimator is used. Friedewald LDL-C is invalid when triglycerides are 400 mg/dL (4.5 mmol/L) or higher. Its accuracy also declines as triglycerides rise and LDL-C falls. The 2026 ACC/AHA multisociety guideline recommends Martin/Hopkins or Sampson/NIH estimation for current clinical reporting.5

For either a fasting or nonfasting profile, non-HDL cholesterol requires only total cholesterol and HDL-C in matching units:5

non-HDL-C = total cholesterol − HDL-C

Worked example. A fasting panel has total cholesterol 218 mg/dL, triglycerides 160 mg/dL, and HDL-C 50 mg/dL.

estimated VLDL-C = 160 ÷ 5 = 32 mg/dL

Friedewald LDL-C = 218 − 50 − 32 = 136 mg/dL

non-HDL-C = 218 − 50 = 168 mg/dL

Albumin/globulin ratio

Calculated globulin equals total protein minus albumin from the same specimen and represents the nonalbumin protein fraction. Express both measurements in the same concentration units before calculating the ratio.6

calculated globulin = total protein − albumin

albumin/globulin ratio = albumin ÷ calculated globulin

Because the concentration units cancel, the ratio has no unit.

Worked example. Total protein is 7.5 g/dL and albumin is 4.2 g/dL.

calculated globulin = 7.5 − 4.2 = 3.3 g/dL

albumin/globulin ratio = 4.2 ÷ 3.3 = 1.2727, reported as 1.27

Use unrounded component values when they are available, then apply the laboratory’s display convention.

Absorbance and percent transmittance

The Beer-Lambert law relates absorbance to concentration and optical path length. With specific absorptivity a, path length b, and concentration c:

A = a × b × c

Absorbance A is dimensionless. When a is expressed in L·mg−1·cm−1, b is in centimeters, and c is in mg/L, their units cancel. Molar absorption uses A = εbc, with molar absorption coefficient ε and molar concentration.6,7

Fractional transmittance T is the ratio of transmitted light to incident light. Multiply T by 100 to obtain percent transmittance:

%T = 100 × T

A = −log10(T) = 2.00 − log10(%T)

T = 10−A

%T = 10(2.00 − A)

In the logarithmic equation, %T is the numerical percentage from 0 to 100. The Beer-Lambert relationship is valid only under the sample and wavelength conditions established by the method. Turbidity, stray light, chemical changes, and measurements outside the validated linear interval can alter the relationship.6,7

Worked example. A solution has a specific absorptivity of 0.0120 L·mg−1·cm−1, a path length of 1.00 cm, and a concentration of 25.0 mg/L.

A = 0.0120 × 1.00 × 25.0 = 0.300

%T = 10(2.00 − 0.300) = 50.1%

References

  1. Mayo Clinic Laboratories. Iron and Total Iron-Binding Capacity, Serum. Accessed August 28, 2026.
  2. Yamanishi H, Iyama S, Yamaguchi Y, Kanakura Y, Iwatani Y. Total iron-binding capacity calculated from serum transferrin concentration or serum iron concentration and unsaturated iron-binding capacity. Clin Chem. 2003;49(1):175-178. doi:10.1373/49.1.175.
  3. Mayo Clinic Laboratories. Bilirubin, Direct, Serum. Accessed August 28, 2026.
  4. Friedewald WT, Levy RI, Fredrickson DS. Estimation of the concentration of low-density lipoprotein cholesterol in plasma, without use of the preparative ultracentrifuge. Clin Chem. 1972;18(6):499-502. doi:10.1093/clinchem/18.6.499.
  5. Blumenthal RS, Morris PB, Gaudino M, et al. 2026 ACC/AHA/AACVPR/ABC/ACPM/ADA/AGS/APhA/ASPC/NLA/PCNA guideline on the management of dyslipidemia: a report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines. Circulation. 2026;153:e1154-e1276. doi:10.1161/CIR.0000000000001423.
  6. Bishop ML, Fody EP, Van Siclen C, Mistler JM, Moy M. Clinical Chemistry: Principles, Techniques, and Correlations. 9th ed. Jones & Bartlett Learning; 2023.
  7. International Union of Pure and Applied Chemistry. Beer-Lambert law. In: Compendium of Chemical Terminology. 5th ed. Online version 5.0.0. 2025. Accessed August 28, 2026. doi:10.1351/goldbook.B00626.