All children in Scotland should be given a "unique learner number" to track progress, MSPs have been told.
Petra Wend, of the Commission on Widening Access, said two out of three children who are deprived are not picked up by the Scottish Index of Multiple Deprivation (SIMD) - the common measure of deprivation.
She said the statistic is not the "right way" to measure deprivation and the unique learner number would be better, but her commission colleague Maureen McKenna said SIMD is "reasonable" and should not be discounted.
They were giving evidence to the Education and Skills Committee as it examines widening access to university, including looking at recommendations on contextual admissions - where students with lower grades than required may be admitted to a university course due to mitigating factors.
The Scottish Government has set a target for one fifth of students entering university by 2030 to be from Scotland's 20% most deprived communities, measured by SIMD.
Ms Wend said: "We did realise that SIMD is not the right way to measure deprivation because two out of three live in non-SIMD 20 areas.
"This is why one of our recommendations is there should be a unique learner number allocated to each child whereby we could actually follow the progression of the child through life in order to measure long-term where interventions can succeed."
Questioned if the target in combination with capped places for Scottish students would end up excluding able students, she said it could hit the "squeezed middle".
She said: "We decided it wasn't up to us to make recommendations as to whether student numbers are capped or not, it was up to the Scottish Government to react to our recommendations.
"Whether displacement is taking place or not is arguable but it is clear if by 2030 we want 20% of SIMD 20 students in universities, it means that there are some students who would have got in who might not get in.
"The question whether that is fair or not is a different matter, because somebody who would have got in now with particular grades might show less potential than a student who wouldn't have got in now and who would actually succeed.
"It's not right to say we need another 20% places, but there is a danger that some students might be squeezed out and it might be those who are just about getting into university, so the squeezed middle."
She said the issue is being looked at "very carefully".
Commission chairwoman Dame Ruth Silver added: "You have displacement now. You have displacement of bright young people who don't have the right badges through no fault of their own.
"Scotland, in the four nations, has got the highest percentage of advantaged young people going into university than the other nations."
Appearing at the committee later, Professor Peter Scott, Commissioner for Fair Access, said the SIMD is a "relatively comprehensive" measure which takes into account multiple aspects of deprivation.
"I think there are strong links... between areas of deprivation and lower performing schools," he said.
"I am very aware that any kind of area-based metric, there are issues.
"The work that Universities Scotland is undertaking looking at this area, and frankly the scepticism that I think Universities Scotland has about how appropriate this is as a measure to determine targets, I think that's to be welcomed.
"I think any work that can be done to develop a more sophisticated basket of measures should be encouraged."
A Scottish Government spokesman said: "The number of fully-funded places available in our universities has increased for 2016/17 by 2,222 - enabled by over £1 billion investment from the Scottish Government.
"The draft Budget for 2017-18 again features direct Government investment of over £1 billion in our higher education sector.
"The concept of a unique learner number is an interesting one that can be explored as part of our work to improve the learner journey for every young person, and implementation of the recommendations made by the Commission on Widening Access.
"In leading the implementation process, the new independent Commissioner, Professor Peter Scott, will consider which measures, in addition to SIMD, will be most helpful in achieving greater fairness in access to higher education for all."
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