FRANKFURT, April 30 (Reuters) - Shares of laboratory services group Synlab made its market debut in the Frankfurt stock exchange on Friday, opening flat at 18 euros ($21.80) per share, equal to its IPO price. Synlab had cut the size of its IPO and priced the shares at the low end of the marketing range after investors questioned the sustainability of earnings, which had spiked in 2020 on the back of strong demand for Synlab’s COVID-19 testing capacities. The deal gives Synlab a market capitalisation of 4 billion euros, making it one of Frankfurt’s largest listings in a busy season for stock market flotations that has included Vantage Towers and used-car trading platform AUTO1 . The company offered new shares worth 400 million euros plus existing shares worth 271 million euros, excluding an overallotment option. If the greenshoe option is fully exercised, Synlab’s free float will amount to 19%. Synlab is owned by buyout fund Cinven, Novo Holdings, Ontario Teachers’ Pension Plan Board as well as Synlab founder Bartl Wimmer. ($1 = 0.8258 euros) (Reporting by Arno Schuetze Editing by Caroline Copley)