One Number, Measured Correctly

Abstract

Rationale: Immunoassays quantify a hormone indirectly — by how strongly an antibody binds it. That proxy is vulnerable to cross-reactivity with structurally similar molecules, to matrix effects from binding proteins, and to disagreement between manufacturers. LC-MS/MS instead separates compounds and identifies each by its mass-to-charge signature, measuring the target molecule itself.
Key distinctions: Across cortisol, androgens and 17-hydroxyprogesterone, estradiol, and 25-hydroxyvitamin D, the peer-reviewed record shows immunoassays systematically diverging from the structural method — often reading high enough to trigger unnecessary downstream workups. LC-MS/MS tracks certified reference materials more closely and reduces between-laboratory variation.
Clinical implications: Where a result informs interpretation near a decision threshold, the assay's specificity is not a technical footnote. It is the difference between a number a clinician can act on and one that may send a patient down an unnecessary path.
Conclusion: The documented cost of LC-MS/MS is operational — lower automation, method-specific reference intervals, heavier quality-control demands — and it falls on the laboratory that runs it, not on the clinician who orders it.

Keywords:LC-MS/MS · immunoassay · analytical specificity · cross-reactivity · steroid measurement · assay standardisation

1. The measurement problem, stated plainly

An immunoassay does not see a molecule; it sees whatever its antibody binds. When two molecules share enough structure, the antibody counts both, and the reported concentration is the sum of the target plus its look-alikes.

A mass spectrometer, by contrast, first separates compounds chromatographically in time, then confirms each by a mass-to-charge transition specific to that molecule.

The practical consequence is a difference in what the number means: an immunoassay result is a binding estimate, an LC-MS/MS result is a structural measurement.^1,2,8

This is not a marginal distinction. A 2024 review of the technology’s clinical evolution frames specificity and sensitivity as the defining reasons LC-MS/MS has spread through reference laboratories, and an operations-focused review reaches the same conclusion on its analytical advantage over antibody-based platforms.^1,2

2. Cross-reactivity is real, measurable, and clinically consequential

Cortisol is the clearest case. Antibody-based cortisol assays cross-react with cortisone and other structurally related metabolites, a problem especially pronounced in saliva and urine.

A dedicated method-comparison review concludes that LC-MS/MS yields more accurate results, considerably reduced variation across laboratories, and avoids false positives — while also enabling tasks immunoassays cannot perform, such as distinguishing exogenous corticosteroids or confirming dexamethasone exposure within a suppression test.³

A companion screening review makes the mechanism explicit: antibody measurement of cortisol generates false positives through cross-reactivity between cortisol, cortisone and other metabolites, whereas structurally based assays measure cortisol alone.⁴

On the left, an antibody binds cortisol together with cortisone and structurally related metabolites, producing a single blended signal in which cross-reactants are counted as the target. On the right, chromatographic separation in time followed by a mass-to-charge transition keeps each analyte distinct, so only the target molecule is quantified. — **Figure 1.** Antibody binding groups look-alike molecules into a single blended signal (left), so cross-reactants are counted as the target. Chromatographic separation followed by a mass-to-charge transition keeps each analyte distinct (right), so only the target is quantified. Schematic, based on cross-reactivity mechanisms described in the cortisol literature.^3,4

3. The bias signature: immunoassays read high

When the two methods are run on the same samples, the divergence is directional.

In women evaluated for androgen excess, immunoassay-measured 17-hydroxyprogesterone ran far above LC-MS/MS — median values of roughly 5.5 and 3.6 nmol/L by two ELISA kits versus 1.6 nmol/L by LC-MS/MS — and testosterone, androstenedione and DHEA-S were likewise overestimated.⁷

The cost was concrete. Unnecessary follow-up diagnostic procedures were undertaken in 85% of patients with one immunoassay kit and 50% with another, and in none when LC-MS/MS was used.⁷

The capability behind that accuracy is chromatographic: LC-MS/MS can resolve isobaric steroids — 21-deoxycortisol, 11-deoxycortisol and corticosterone, for example — that antibodies cannot tell apart.⁸ Clinical guidance on hyperandrogenism has positioned mass spectrometry as the emerging standard over direct immunoassay.⁹

4. Estradiol and the low-concentration ceiling

The Endocrine Society’s position statement on estradiol is the authoritative reference. It concludes that although immunoassay and LC-MS/MS are both adequate for fertility-range work, imprecision and method-to-method differences remain problematic, and the very low estradiol concentrations relevant outside reproduction are frequently too low to be measured accurately or precisely by routine assays.⁵

Method-development work shows where LC-MS/MS extends that ceiling: a derivatisation-free assay reached a limit of quantification of 7.5 pmol/L and correlated closely with a chemiluminescent immunoassay while showing a systematic negative bias against it — that is, the immunoassay again read higher.⁶

Where Clinicore’s own estradiol measuring interval falls, and what it does and does not support, is stated in the estrone and estradiol review.

5. Beyond hormones: standardisation and 25-hydroxyvitamin D

The pattern is not confined to steroids.

Compared against certified reference material SRM 972, only LC-MS/MS results fell close to the certified values; three automated immunoassays diverged in different directions, and that divergence shifted the apparent prevalence of vitamin D insufficiency by more than ten percentage points depending on method.¹⁰

In a large population survey, LC-MS/MS 25-hydroxyvitamin D averaged 12.9 ng/mL higher than a radioimmunoassay on the same samples — a gap large enough to require statistical harmonisation before results could be compared.¹¹

The implication for any test informing a threshold decision is the same: a patient’s classification can hinge on which assay a laboratory happens to run, and LC-MS/MS is the method that tracks the reference standard.

Table 1. Representative method-comparison findings

Analyte	Matrix	Reported finding	Ref.
17-OHP and androgens	Serum	Immunoassay ~2–3.5× higher; unnecessary workups 50–85% versus 0%	7
Cortisol	Saliva, urine, serum	Antibody cross-reactivity with cortisone and metabolites; MS reduces inter-laboratory variation	3, 4
Estradiol	Serum	Low concentrations below reliable routine accuracy; immunoassay positive bias	5, 6
25-OH vitamin D	Serum	Only LC-MS/MS near SRM 972; RIA ~12.9 ng/mL difference	10, 11

6. The known trade-offs, and where they land

A credible account does not pretend LC-MS/MS is free. The same reviews that praise its specificity flag genuine constraints: most LC-MS/MS assays are laboratory-developed rather than turnkey kits, carrying heavier validation, quality-assurance and troubleshooting responsibilities,² and broader adoption is still limited by lower automation, the need for method-specific reference intervals across diverse populations, and immature commercial quality-control materials.¹

Critically, these are laboratory-side costs. They are borne by the laboratory that develops, validates and monitors the assay — not by the clinician who orders it.

Clinicore’s role is to carry that operational weight so what reaches the ordering provider is the analytical benefit on its own: a result that reflects the molecule, not the antibody’s best guess. How a method becomes reportable here is set out on the validation page.

7. What this means for the ordering clinician

When a result will inform interpretation near a decision threshold — an androgen in a woman with suspected excess, a low estradiol, a cortisol read against a suppression test, a borderline vitamin D — the assay’s specificity is not a technical footnote. It is the difference between a number you can act on and one that may send a patient down an unnecessary path.^3,5,7

The thesis of this series follows directly. Measuring a pathway and reading the ratio between two analytes only means something if each analyte is measured accurately. Antibody cross-reactivity corrupts exactly that — a cortisol-to-cortisone ratio is meaningless if the cortisol antibody is also counting cortisone.

Structural measurement is the precondition for every ratio in this series.