Esta frase ayuda a entender el significado de 'baseline' en este contexto:

"A 1998 study reported baseline retinopathy levels were present in 39% of men and 35% of women with newly diagnosed Type 2 diabetes."
http://diabeticeye.screening.nhs.uk/faqs-condition

Se trata, pues, de una retinopatía que ya está presente cuando se diagnostica la diabetes.

Un ejemplo con "retinopatía prexistente":

http://revistasad.org.ar/numeros/2012_46_especial/Guias_para...

y uno con "retinopatía previa":

http://bvs.insp.mx/rsp/articulos/articulo.php?id=002587

Saludos cordiales.

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Note added at 48 minutos (2013-02-03 16:08:53 GMT)
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Más sobre el significado de 'baseline retinopathy' (las negritas las agregué yo para facilitar la lectura):

"Previously, our group developed models to locally predict new retinopathy development in patients with and without baseline retinopathy over a 1- to 3-year follow-up period. We found several factors that are predictive of new retinopathy in these patients, including mfERG IT Z-score, diabetes duration, blood glucose, and the presence of DR at baseline. In our previous 1-year model,24 the strongest factor predicting new retinopathy in a retinal zone was previous retinopathy in the eye. (This is perhaps not surprising because it is clinically accepted that an eye with some retinopathy is at a higher risk for more pathology.) Our previous models did not have enough patients without baseline retinopathy who went on to develop retinopathy in the follow-up period to make predictions about the onset of retinopathy in those eyes. Consequently, in this study, we investigated a group of patients with no retinopathy at baseline over a longer period. This model is a critical step for predicting the first funduscopically obvious change in the retina at the typical interval between diabetic eye examinations. The clinical onset of DR (vasculopathy in the retina) signifies an important progression in the microvascular complications in diabetes, which may also be occurring in other organ systems. This progression can be an indicator for the physician and patient to consider more aggressive management, including shorter patient follow-up intervals."
(En http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3053106/)

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Note added at 75 días (2013-04-19 22:50:58 GMT) Post-grading
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De nada.