Thousands of people in England and Wales are unaware they may have a high genetic risk of heart disease because testing is patchy, doctors warn.
When people are diagnosed with familial hypercholesterolaemia - an inherited condition causing high cholesterol - family members should also be screened.
But an audit by the Royal College of Physicians found genetic testing is not happening in many places.
One expert said up to 80% of those affected are not diagnosed.
Familial hypercholesterolaemia (FH) causes high levels of "bad" or LDL cholesterol.
If left untreated it can lead 50% risk of heart disease in men by the age of 50 years and at least 30% in women by the age of 60 years.
However, early treatment with cholesterol-busting drugs - statins - is effective.
In 2008, the National Institute for Health and Clinical Excellence (NICE), estimated that one in 500 people have FH and siblings or children of a person with the condition have a 50% chance of having it themselves.
They recommended that everyone diagnosed with FH should be referred to a specialist for "cascade testing" where family members at risk are identified and offered screening.
The audit of 248 patients in 12 hospitals in England and Wales found that while care for those diagnosed was generally good, very few of their families were being systematically tested.
Also there were not enough facilities for diagnosing and treating children with FH.
Better screening, as recommended by NICE, would save lives but also save the NHS money in the long run, the Royal College of Physicians said.
To improve standards, NHS Trusts will need additional resources for specialist doctors and nurses and funding for DNA tests, the report concluded.
Professor Steve Humphries, director of the Centre for Cardiovascular Genetics at the British Heart Foundation Laboratories in London and audit leader, said sorting out screening was a "key priority".
"The problem at the moment is that most NHS trusts haven't yet been able to find funding for the DNA testing or the nursing and support staff to carry out the cascade family testing.
Professor Roger Boyle, national clinical director for heart disease and stroke, said the government expected trusts to follow the NICE guidance and offer screening to relatives.
Dr Alan Rees, chairman of Heart UK, the cholesterol charity, said: "We fail to diagnose 80% of people with FH in the UK.
"What clearly hasn't been happening is the testing of every family member of someone with FH which is essential."
Dr Mike Knapton, from the British Heart Foundation, said delaying investment in screening was cutting lives "needlessly short".
"There are around 100,000 people going about their lives unaware they are at risk of this silent killer - people who, with the right treatment and management could have a normal life expectancy.