Surrogate Outcomes

Why it matters

Outcomes people care about are often rare or slow, so investigators measure something faster and commoner instead — a marker standing in for the outcome. Substitution generally costs certainty, and GRADE restricts its use rather than merely asking that it be recorded: surrogates are for cases where evidence on population-important outcomes is lacking. [EXTRACTED (GRADE - Handbook) §3.4]

The rule

GRADE’s conditions, stated tightly:

• Consider surrogates only when evidence about population-important outcomes is lacking — not as a first-line convenience.
• When one is used, specify the population-important outcome and, separately, the surrogate substituting for it.
• “Guideline developers should not list the surrogates themselves as their measures of outcome.” The outcome remains the thing that matters; the surrogate is a route to estimating it.
• Substituting “may ultimately lead to rating down the quality of the evidence because of the indirectness”; §5.2.3 puts it more firmly — “in general, the use of a surrogate outcome requires rating down … by one, or even two, levels.” [EXTRACTED (GRADE - Handbook) §3.4, §5.2.3]

How GRADE grades a surrogate — causal-pathway proximity

The first of two criteria the handbook supplies, and the most decision-relevant thing it says about surrogates: the penalty scales with distance along the putative causal pathway to the patient-important outcome.

• Far from the endpoint -> rate down two levels. GRADE’s case is calcium and phosphate metabolism standing in for patient-important outcomes in renal disease.
• Close to the endpoint -> rate down one level. Coronary artery calcification for myocardial infarction; bone mineral density for fractures; soft-tissue calcification for pain.
• The judgment draws on “consideration of the biology, mechanism, and natural history of the disease.” [EXTRACTED (GRADE - Handbook) §5.2.3]

So a marker’s standing is not binary. How far down the chain it sits is the question, and it is answered biologically rather than statistically.

The second criterion — does the chain actually transmit?

Proximity is not the only test the handbook supplies. Immediately after the proximity passage it adds a validation criterion, addressing whether the surrogate predicts the outcome at all:

“Example 9: Uncertainty in the relationship between surrogate and Surrogate outcomes (Downgraded by One or Two Levels) — Investigators examined the ‘validity’ of progression-free survival as a surrogate for overall survival… They found a statistically significant association between progression-free and overall survival in the randomized trials they analyzed, but predicting overall survival using progression-free survival remained uncertain. Rating down quality by one level for indirectness would be appropriate in this situation.”

[EXTRACTED (GRADE - Handbook) §5.2.3, Example 9]

This is the criterion that does the work the proximity rule cannot. A significant marker-outcome association was not sufficient — what GRADE asks is whether the outcome can be predicted from the marker, which is a strictly stronger demand than correlation. A marker can be proximate, correlated, and still fail here.

Two further treatments elsewhere in the handbook point the same way: that “even if well measured surrogates are available, confidence in estimates of effects on patient-important outcomes is very likely to be low”, and a dedicated list of “Key questions when using test accuracy as a surrogate”.

Tests / indicators

• Is the outcome named separately from the marker? A recommendation phrased directly in terms of a marker (lower X to below Y) has collapsed the distinction and cannot be checked.
• Was certainty rated down for indirectness — and if not, is a reason on the record? GRADE’s rule is “in general”, with an explicit judgment clause, so a non-downgrade is not automatically a defect. Three cases, and only the first two are: the surrogate was silently treated as the outcome; the downgrade was skipped with nothing said; or a stated, biology-based judgment was made that the chain transmits. The third is what GRADE invites — WHO’s LDL case below is a worked instance. The test is whether a reason was given, not whether the downgrade was taken.
• Is the marker -> outcome link itself evidenced? GRADE places surrogate use under indirectness, which presumes the link is an inference rather than an observation. The strength of that link is the whole question, and it is a separate evidential claim from the intervention -> marker effect.

Red flags

• A marker used because it is measurable, where the patient-important outcome was never named (the failure Rating Outcome Importance guards with the empty-row rule).
• A target expressed as a threshold on a marker, with no statement of which outcome it is a route to.
• Certainty ratings that look high for a body of evidence entirely composed of marker studies.

Green flags

• The evidence profile names the patient-important outcome, shows the surrogate used for it, and carries an explicit indirectness downgrade.
• The direction and magnitude of the marker -> outcome transmission is cited, not assumed.

Decision relevance

A surrogate may legitimately serve as a target to steer toward even where it is a weak stand-in for the outcome in the evidence — but only if the marker -> outcome transmission is itself an evidenced claim. Where it is assumed, the recommendation inherits the assumption silently, and markers have moved in the intended direction while the outcomes did not.

Two consumer-facing interventions that live entirely on this line: glucose tracking (Continuous Glucose Monitoring as a Health Intervention — moves HbA1c modestly, no hard outcome, almost no data in the healthy) and low-AGE / low-heat cooking (Dietary AGEs and Cooking Method — moves some lipid/inflammation markers, null on weight/glucose/BP, no hard outcome). Both are marketed on a marker whose transmission to a patient-important outcome is unevidenced.

The validated counter-exemplar — LDL / apoB. The rule cuts both ways: a surrogate whose causal transmission is evidenced is a legitimate target. LDL/apoB-particle burden is the strongest such case the wiki holds — genetic, Mendelian-randomization and RCT evidence together establish it causes ASCVD, so lowering it (by diet or drug) reduces events in proportion to the reduction achieved -> LDL ApoB and Cumulative Exposure. It is the opposite of the “marker moved, patient did worse” cases: not all surrogates are equal, and this one has earned target status. (The proviso still bites — the transmission holds only where the LDL-C drop reflects a real particle-number drop and carries no off-target harm.)

Applied — WHO downgrades strength because the evidence is a surrogate

WHO’s 2023 fat guideline supplies a worked instance of a surrogate costing strength, not just certainty. For replacing trans-fatty acids, the evidence runs through LDL cholesterol, and WHO reasons: while LDL “is a well-established biomarker for measuring the effects of interventions on CVD risk, and is considered by many to be a causal factor for atherosclerosis and coronary heart disease, it is not a physical manifestation or confirmation of disease. Therefore, a conservative approach was taken, leading to a conditional recommendation.” [EXTRACTED (WHO - Saturated and Trans Fatty Acid Intake 2023) Rationale for TFA recommendation 3]

Two things worth carrying:

• The marker was granted almost every credential a surrogate can have — well-established biomarker, widely held to be causal — and still produced a weaker recommendation. Surrogate status is not cancelled by a plausible mechanism.
• WHO explicitly declined the indirectness downgrade — and that is a live tension. It records LDL as a critical outcome and states it “was not downgraded for indirectness when determining the certainty in the evidence within the GRADE framework”. The surrogate discount was taken at the strength step instead. [EXTRACTED (WHO - Saturated and Trans Fatty Acid Intake 2023) Annex 6 footnotes; Summary of evidence]
  • WHO’s warrant is the MARKER-to-OUTCOME link, not the exposure-to-marker one, and the direction matters because it is what criterion 2 above asks for. Annex 6 fn 14: “LDL cholesterol is an indirect measure of patient-important CVD outcomes. However, LDL cholesterol is a well-established biomarker for assessing the effects of interventions on CVD risk…, and is considered by many to be a causal factor for atherosclerosis and coronary heart disease. Therefore not downgraded for indirectness.” WHO is asserting that the chain transmits — the exact question Example 9 poses — not that SFA reliably moves LDL.
  • So this is NOT the tension it looks like, and an earlier reading of it as one does not survive. GRADE’s rule is “in general, the use of a surrogate outcome requires rating down”, and it attaches an explicit judgment clause (“Consideration of the biology, mechanism, and natural history of the disease can be helpful in making a decision about indirectness”). WHO exercised that judgment, on the transmission question, with its reason on the record. A reasoned, documented, biology-based decision not to downgrade is the judgment GRADE invites, not a departure from a mandate — the not-joined check fires, and what is recorded here is a distinction, not a [[tension]].
  • What remains genuinely notable is narrower and still worth holding: the same marker was judged direct enough to escape the certainty downgrade and indirect enough to cost strength. WHO’s stated reason for the second is that LDL “is not a physical manifestation or confirmation of disease” — which is true of it at the certainty step as well. The two steps were held to different standards, and only the strength step’s standard is stated.
• The evidence on the surrogate can be stronger than the evidence on the outcome and still yield a weaker recommendation: in the same guideline, replacing SFA with PUFA, MUFA or carbohydrate lowers LDL at high certainty, while the hard-outcome certainty behind those replacements is moderate to low (Saturated Fat Intake and Replacement). High certainty about the marker is not high certainty about the person.

The certainty profile of a surrogate vs its own hard outcomes (2026-07-26)

A scoped observation, worth carrying because it is visible only in the annex. In WHO’s evidence profile for replacing SFA with PUFA, every disease outcome is rated Low or Very low, and the one High-certainty row in the profile is LDL cholesterol (-0.055 mmol/L per 1% energy exchange; all-cause mortality Low, CVD mortality Very low, CVDs Low, CHD Low, stroke Low, type 2 diabetes Very low). [EXTRACTED (WHO - Saturated and Trans Fatty Acid Intake 2023) Annex 6, evidence profile 5]

The disciplined reading, and the trap beside it. The useful habit is to ask which outcome carried a headline certainty label. The trap is to answer that question from the profile alone: WHO states elsewhere that its overall certainty “was based on disease and mortality outcomes”, and draws its moderate rating for the PUFA replacement from an RCT subgroup analysis (Hooper) that sits outside this profile. A first attempt at this page’s neighbourhood asserted the surrogate had silently carried the roll-up; a blind critique falsified it against that sentence, and the claim is withdrawn. What survives is the observation above and the habit — not an allegation. [EXTRACTED (WHO - Saturated and Trans Fatty Acid Intake 2023) Summary of evidence]

Limits

• GRADE’s two criteria are stated but not operationalized. Proximity is coarse and biologically judged; the Example 9 validation criterion asks the right question — can the outcome be predicted from the marker — but is delivered as a worked case rather than a standard, with no threshold for how much predictive uncertainty triggers one level versus two. A quantitative surrogate-validation framework (trial-level and individual-level association measures) would convert both into checkable standards; whether one exists that guideline bodies actually apply is unprobed here.
• Source currency: §3 is flagged in-source as rewritten in the 2024 GRADE Book.

A guidance-level version of the same critique [2026-07-28, Willett ch.16]

This page asks when a recommendation may rest on a marker. Willett’s policy chapter makes the same objection about how nutrition guidance has historically been built:

«Even until now, many dietary recommendations and policies have been based on short-term feeding studies with different levels of an essential nutrient and a single physiological variable as the outcome. While these can be important, such studies examine the effect of the dietary factor through a very narrow window because they do not consider the many pathways by which a dietary factor can influence health and the many steps in a pathway from dietary intake to occurrence of disease.» [EXTRACTED (Willett - Nutritional Epidemiology 3e) chunk 21]

Note the concessive clause — «While these can be important» — is Willett’s, not a hedge added here. He is not rejecting feeding studies; he is bounding what a single physiological endpoint can carry.

The structure of the objection is precisely this page’s: a short-term feeding study measures one step in a causal chain, and a recommendation asserts something about the endpoint. The gap is the number of unmeasured steps and unconsidered pathways, which is why a marker can move correctly while the outcome does not.

Where this lands in the corpus. The wiki already holds WHO’s LDL non-downgrade as a worked case of a body reasoning across that gap explicitly. Willett’s claim is the general version, from a methods textbook rather than a guideline — so it is a second, differently-situated statement of the same concern, and unlike the WHO case it is not one body assessing its own process.

Bounded, because Willett is a participant in these debates and not a neutral observer of them. The same chapter asserts that «for years dietary guidelines have emphasized low-fat, high-carbohydrate diets when there was little evidence of benefit» — a substantive and contested position on a question this wiki has not adjudicated. Recorded as Willett’s stated position, not adopted; it bears on SC-12, and the handle is noted here without scoring it, per the ingest-scope rule. The surrogate point above stands on its own reasoning and does not depend on that claim being right.

The strongest surrogate-to-TARGET argument the corpus holds [2026-07-28, ESC]

The telos distinguishes a surrogate (stands in for an outcome in the evidence) from a target (something to steer toward), and licenses a target only if its causal transmission to a named patient-important outcome is itself an evidenced claim. ESC - CVD Prevention Guidelines 2021 makes exactly that argument for LDL-C, and its structure is worth extracting whatever one concludes about the conclusion.

ESC’s preamble — its own confidence claim, recorded as ESC’s and not adopted: the causal role of LDL-C in ASCVD «is demonstrated beyond any doubt by genetic, observational, and interventional studies».

Then the load-bearing attribute:

«The relative reduction in CVD risk is proportional to the absolute size of the change in LDL-C, irrespective of the drug(s) used to achieve such change.» [EXTRACTED (ESC - CVD Prevention Guidelines 2021) chunk 02]

Why «irrespective of the drug» is the whole argument. A marker is a mere surrogate when the benefit might belong to the intervention rather than to the marker’s movement. If several drugs with different mechanisms produce outcome benefit proportional to the LDL-C change they achieve, the common factor is the marker, not any drug’s off-target effects. That is a dose-response test across causal routes — the marker earns target status by behaving the same way no matter how it is moved.

This is the same discriminator Willett’s case studies identified, arriving in a different domain: convergence across independent method classes (here genetic, observational, interventional) is what separated folic acid from vitamin A and dietary fat -> Upgrading Observational Evidence.

Three bounds, because this page’s job is scepticism about markers.

• The wiki has NOT verified the underlying claim. ESC asserts drug-independence with one reference; this corpus holds none of the trials, no Mendelian-randomisation source, and no contrary analysis. What is recorded is the argument’s structure and ESC’s assertion, not a verified fact about LDL.
• «Beyond any doubt» is a guideline’s rhetoric, not a certainty rating. ESC attaches no GRADE-style certainty to it, and this corpus has repeatedly found strength language and evidence grading to move independently -> Certainty of Evidence vs Strength of Recommendation.
• A validated target for one outcome is not a validated target generally. The claim is about ASCVD. It licenses nothing about LDL-C and all-cause mortality, cancer, or any other endpoint, and the telos’s warning stands: markers have moved the right way while patients did worse.

Contrast worth keeping, and it is what makes this page’s distinction operational. WHO’s LDL non-downgrade (above) was a judgement that indirectness did not apply. ESC’s argument is a different kind of thing — an empirical claim about invariance across interventions, which is checkable. Two bodies, two routes to treating LDL as more than a surrogate: one by reasoned non-downgrade, one by asserted mechanism-independence. The second is the stronger form, and the corpus should hold the sources that test it. AWAITS a Mendelian-randomisation or trial-pooling source on LDL-lowering across drug classes.

The certainty gradient runs opposite to outcome importance — measured twice [2026-07-28]

WHO’s SFA Annex 6 grades every outcome for one exposure. The ordering is the finding:

Outcome class	Best certainty in the annex
LDL cholesterol (surrogate)	High — the only High row
Cardiovascular events (hard, composite)	Moderate
All-cause mortality (hard)	Moderate
CVD / CHD mortality, type 2 diabetes	Low
Stroke, CHD events	Very low

[EXTRACTED (WHO - Saturated and Trans Fatty Acid Intake 2023) Annex 6, pp.79-80]

The best-known quantity is the one nobody cares about directly, and the outcomes a person would actually choose on are known least well. This is not a defect in WHO’s grading — it reflects that a lipid response can be randomised, measured in weeks, in small samples, while an event outcome cannot. The certainty gradient tracks measurability, not importance.

And it is now measured twice, on two exposures, in two guidelines. Sodium Intake and Blood Pressure has the identical shape: High certainty on blood pressure, very low on hard outcomes. Two independent exposures showing the same inversion is the beginning of a general claim about this literature — that wherever a nutrient acts through a measurable intermediate, the intermediate will be better graded than the endpoint, structurally and permanently, because the designs that grade well are the ones the intermediate admits. [INFERRED (WHO - Saturated and Trans Fatty Acid Intake 2023; WHO - Sodium Intake 2012) — both gradients are WHO's; the generalisation is this page's]

Why this is the practical core of the surrogate problem. The pressure to act on the marker is not irrationality — it is a rational response to the marker being the better-evidenced quantity. A reader following “act on the strongest evidence” is led toward LDL and blood pressure by the grading itself. The discipline this page asks for therefore runs against the grain of the evidence hierarchy, not with it, which is why it needs stating rather than assuming.

The guard that keeps this honest: two exposures is two, and both are WHO’s. A third body grading the same inversion on a different exposure would make this a property of the literature; two WHO guidelines make it a hypothesis with two instances.

The mirror case — a surrogate WHO DECLINED to credit [2026-07-29, WHO NSS 2023]

The LDL and ESC cases above are surrogates argued up to target status. Non-Sugar Sweeteners is the opposite move by the same body: a moving surrogate WHO refused to bank. In the NSS trials, short-term body weight falls (MD -0.71 kg pooled), yet WHO ruled that «evidence of minor weight loss or reduced BMI over several months or less … does not represent a health benefit», because weight «must be sustained over the long term» to matter — and the long-term cohorts point the other way. [EXTRACTED (WHO - Non-Sugar Sweeteners 2023) chunk 01]

The same second-criterion test, opposite verdict — read through this page’s lens. This page’s load-bearing test (Example 9) is whether the outcome can be predicted from the surrogate — whether the chain transmits. WHO did not cite Example 9 for either exposure, but its stated reasons map cleanly onto that test, and land oppositely for two of its own surrogates:

Case	Surrogate	WHO’s stated reason (mapped to transmission)	WHO’s verdict	Same quantity?
SFA/TFA 2023	LDL cholesterol	«considered by many to be a causal factor for atherosclerosis» — transmission asserted (genetic/MR/RCT elsewhere)	credited — declined indirectness downgrade	—
NSS 2023	short-term body weight	short-term loss «does not represent a health benefit» absent evidence it is «sustained over the long term» — no evidence the marker predicts the outcome	declined — not banked as benefit	No — different surrogates, one rule

Why this is a refinement, not a tension (not-joined check (ii): different surrogates). The two are different quantities judged under the same rule — credit a surrogate only where its transmission to the patient-important outcome is evidenced. LDL cleared it; short-term weight did not. So the NSS case is the negative worked instance of this page’s decision rule: the discipline the page asks for, applied by a guideline body against itself. NON-independent (both WHO) — this corroborates the rule, not via an independent field, so no [E-independent] is claimed.

The sharper point: the surrogate here was not merely weak — it was moving in the good direction while the hard-outcome cohorts moved the other way (weight down in RCTs; obesity/T2D/CVD/mortality associations up in cohorts). That is the textbook marker moved the right way while patients did worse warning made concrete — and it is why a moving surrogate is bankable only when transmission is shown, not assumed. The cohort direction is itself unadjudicated (-> The U-Shaped Association Artifact).