Core Chemistry Calculations

Iron capacity and saturation

Total iron-binding capacity (TIBC) can be calculated from serum iron and unsaturated iron-binding capacity (UIBC) or derived from a transferrin measurement. In Mayo Clinic Laboratories' current SFEC procedure (test ID SFEC, Iron and Total Iron-Binding Capacity, Serum), transferrin is measured turbidimetrically with Roche TRSF2 reagent and TIBC is calculated as transferrin multiplied by 1.18. Use 1.18 only when the validated procedure specifies this laboratory-specific calculation. The catalog does not establish 1.18 as a factor for every Roche TRSF2 application. The theoretical binding relationship is approximately 1.40 to 1.41 µg/dL TIBC per mg/dL transferrin.^{1,21 Mayo Clinic Laboratories. Iron and Total Iron-Binding Capacity, Serum.2 Yamanishi H, et al. Total iron-binding capacity calculated from serum transferrin… 2003.}

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

The factor has units of µg iron per mg transferrin. Convert transferrin from g/L to mg/dL by multiplying by 100 before using this equation: 3.00 g/L becomes 300 mg/dL.

For a transferrin result of 300 mg/dL:

Use the factor specified in the validated procedure. Trace Elements covers iron physiology and interpretation.

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

TIBC = serum iron + UIBC

UIBC = TIBC − serum iron

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

For serum iron of 30 µg/dL and TIBC of 480 µg/dL:

UIBC = 480 − 30 = 450 µg/dL

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

Bilirubin fractionation

Use total bilirubin and the method-defined direct-reacting result from the same specimen and in the same units. Heme Synthesis, Porphyrias, and Bilirubin covers the fraction definitions, method limits, and interpretation.^{33 Mayo Clinic Laboratories. Bilirubin, Direct, Serum.}

calculated indirect bilirubin = total bilirubin − direct bilirubin

With total bilirubin of 10.8 mg/dL and direct bilirubin of 2.4 mg/dL:

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

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 divisor depends on the unit system.^{44 Friedewald WT, et al. Estimation of the concentration of low-density lipoprotein… 1972.}

The original equation used fasting specimens. Lipids, Lipoproteins, and Dyslipidemia covers current estimator selection, method limits, and interpretation.^{4,54 Friedewald WT, et al. Estimation of the concentration of low-density lipoprotein… 1972.5 Blumenthal RS, et al. 2026 ACC/AHA/AACVPR/ABC/ACPM/ADA/AGS/APhA/ASPC/NLA/PCNA… 2026.}

For either a fasting or nonfasting profile, non-HDL cholesterol requires only total cholesterol and HDL-C in matching units:^{55 Blumenthal RS, et al. 2026 ACC/AHA/AACVPR/ABC/ACPM/ADA/AGS/APhA/ASPC/NLA/PCNA… 2026.}

non-HDL-C = total cholesterol − HDL-C

Suppose 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

Use total protein and albumin from the same specimen and express both in the same concentration units. Plasma Proteins and Electrophoresis covers the measurement methods and interpretation.^{66 Bishop ML, et al. Clinical Chemistry. 2023.}

calculated globulin = total protein − albumin

albumin/globulin ratio = albumin ÷ calculated globulin

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

A zero calculated globulin makes the ratio undefined. A negative calculated globulin means the measured albumin exceeds total protein. Check the component results, units, and method interference before reporting an albumin/globulin ratio.

Suppose 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.^{6,76 Bishop ML, et al. Clinical Chemistry. 2023.7 International Union of Pure and Applied… Beer–Lambert law. 2025.}

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 greater than 0 and up to 100. The Beer-Lambert relationship applies under the wavelength, path-length, sample, and concentration conditions established by the method. Quantitative Instrument Measurement Principles covers calibration and departures from linear response.^{6,76 Bishop ML, et al. Clinical Chemistry. 2023.7 International Union of Pure and Applied… Beer–Lambert law. 2025.}

Suppose 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%