The phrase "a true surrogate" is correct and usable in written English.
It can be used to describe someone or something that genuinely fulfills the role or function of a surrogate, often in contexts related to parenting, representation, or substitution.
Example: "In this study, we found that a true surrogate can provide the emotional support needed for children in foster care."
Alternatives: "an authentic substitute" or "a genuine representative."
Secondly, it requires that, conditionally on X, W provides no additional information about the hazard of failure; that is, all of the effects of W on failure and censoring are mediated through X, which is somewhat restricted since W may not be a true surrogate and depends on the failure given X.
Although this prognostic study was not specifically designed to validate FT as a true surrogate endpoint of the severity of HBV chronic hepatitis [38], we observed as in HCV chronic hepatitis that FT fulfilled almost all of the 13 criteria of a surrogate endpoint biomarker [21], [39], including specificity and sensitivity for fibrosis [10], [20] [22].
Importantly, this approach, without the entanglement of the oxidant, would make a C C triple bond in the presence of an oxidant as a true surrogate of the hazardous α-diazo carbonyl moiety in gold catalysis.
The number of CTC pre- and post-chemotherapy above and below a set threshold has proved to be a true surrogate marker for response and survival in advanced breast cancer (Cristofanilli et al, 2004) and trials are currently ongoing investigating this technology in prostate cancer.
Nevertheless, to be useful as true surrogate markers of disease, both types of markers have to reflect disease outcome in a broad sense; namely, a 'meaningful endpoint of how a patient feels, functions and survives' [ 10].
Sarkar and Margules (2002) discuss the role of biodiversity surrogates, arguing that even the relative concept of complementarity has a "conventional" element built into it because it relies on "estimator" surrogates (say, a set of butterfly species) for "true" surrogates (say, the use of species as the basis for assessing complementarity of places).
Thus, there is a pressing need for predictive and pharmacodynamic biomarkers and for those that are true surrogates of clinical response.
Although several biomarkers associated with angiogenesis measured before treatment have shown to provide prognostic value, and a few biomarkers are pharmacodynamic, there is minimal information on biomarkers that are true surrogates for clinical response to VEGF inhibitors.
Whereas estimator surrogates, they argue, are subject to empirical justification, true surrogates are still dependent on convention.
However this creates a new problem of increased numbers of individuals being identified as surrogates that are not in fact true surrogates.
However, clinical decisions based on such evidence should be made cautiously and limited to the use of true surrogates (those that correlate with clinical outcomes).
