"key indication" is correct and usable in written English.
It is usually used to refer to a sign or signal that is important for understanding something or making a decision. For example, "The key indication that the job market is improving is the steady increase in new job postings."
"The hepatitis C is really the key indication," said Jean-Pierre Sommadossi, chief executive of Idenix.
A key indication of how Rousseff, an economist by training, plans to turn this around will be her choice of finance minister.
James does not need a medal to prove she is back on the right track, as the key indication is that she can still cut the mustard at world level and maintain her progress towards Rio, but the team will be hoping nonetheless.
There are three main developments that are tracked in the 'Tanner stages' (named after the British paediatrician that first researched them): for girls it's breast development, for boys it's genital development and for both sexes, pubic hair is a key indication that puberty is underway.
The type of response is a key indication of the action of the toxin.
According to Schain (2010), this is the key indication of the failure of immigration policy to take off at the European level.
The Warwickshire town fell to the Conservatives in 2010, and they held on to it at last year's general election, in one of the key indications that Ed Miliband's party was heading for defeat.
All these could represent key indications to reinforce or refute evidence of a functioning petroleum system, that is, one that generated, migrated and accumulated hydrocarbons.
While Android is still the overall leader in smartphone marketshare, there are several key indications that the iPhone has significantly closed the gap — or perhaps even taken the lead in countries like the U.S. — in the past couple of quarters.
Goldacre actually provided, in his talk, several key indications of scientists deviating from the dictates of operating rationally.
The overtures from Rouhani, and key indications from supreme leader Ayatollah Ali Khamenei that he supports them, have Iran analysts on the lookout for an answering gesture from the White House.
