LONDON (AP) — At the midway point of the London Olympics, no competitor has been caught doping at their event. At least, not yet.

Still, every day a different name of a suspected offender has emerged because of an extensive pre-Games testing program to weed out drugs cheats before they get near a venue.

On Saturday, a Colombian runner, a Brazilian rower and a Moldovan hammer thrower all made minor headlines — and all were barred from starting their event.

Each case at this stage is judged a success in keeping the London Olympics clean.

From now on, every positive doping result from an in-competition sample becomes more problematic for the Olympic image.

Disqualifications follow, often requiring athletes to be stripped of their medals and results to be redrawn. Then, the possibility of lengthy legal action as the field of play switches to the Court of Arbitration for Sport in the International Olympic Committee's home city of Lausanne, Switzerland.

For now, the IOC's legal slate is clean.

"We have shown that we take swift action, that cheats are caught and ejected from these games," spokesman Mark Adams said. "I would say at this stage, it's a pretty low number."

Four is low. That's the number of cases requiring IOC sanctions from testing since the athletes' village officially opened July 16. They are the Colombian, 18-year-old 400-meter runner Diego Palomeque (substance not identified); Russian track cyclist Victoria Baranova (testosterone); gymnast Luiza Galiulina of Uzbekistan (furosemide); and Albanian weightlifter Hysen Pulaku (stanozolol).

The World Anti-Doping Agency zeroed in on a higher number. In the week the Games opened, WADA said that 107 athletes were removed from the picture by suspensions imposed in the first six months this year.

Then add a few more July cases, involving new positive tests and old appeals resolved, which removed genuine medal contenders from the track and field program: in men's discus, hammer and high jump, and the women's 1,500 meters.

Those athletes took stanozolol — better known as the steroid Ben Johnson took at the 1988 Seoul Olympics — the classic blood-booster EPO and, in the case of Hungarian former discus silver medalist Zoltan Kovago, flat out refusing to cooperate with testing teams.

The numbers suggest WADA and the International Association of Athletics Federations got their long-term strategy right.

Every athlete at the 2011 World Championships in Daegu, South Korea, had to provide a blood sample to help WADA laboratories set baseline levels for future monitoring and targeted follow-up tests.

It formed part of a wider program of sharing intelligence between WADA, international governing bodies and the British authorities — anti-doping and law enforcement agencies — trying to make life tougher for athletes intending to illegally enhance their performance on the world stage.

"Doping athletes should know that their chances of avoiding detection are the smallest they have ever been," WADA President John Fahey promised IOC members three days before the London Games opened.

It marked a chance of rhetoric from the global anti-doping watchdog which has often lamented since the 2008 Beijing Games that only the "dopiest dopers" were being caught.

WADA has sought greater government help to enforce its modern mantra of "quality not quantity" in drug-testing, though the London Olympics appears to have both.

The IOC has pledged to take more than 5,000 urine and blood samples during the four-week official Games period. The samples are tested at a dedicated Olympic laboratory working round-the-clock under the supervision of WADA-accredited researchers from King's College, London.

On Saturday morning, Adams said the lab had 2,905 samples, with many still being analyzed.

A doping scandal will be hard to avoid in the coming days.

The 2004 Athens Games produced a record 26 cases at the time, and the number is still rising. Men's hammer silver medalist Ivan Tsikhan was sent home from London to Belarus on Friday because of suspicions arising from reanalysis of his Athens sample.

Six Beijing medals were reallocated because of doping, including the signature men's 1,500 meters gold. Rashid Ramzi of Bahrain was stripped when retests months later found a new version of EPO, called CERA.

London samples will be stored for eight years to allow the IOC to re-examine samples if new technologies and tests allow for deeper analysis.

So far in London, the biggest doping discussion has centered on unfounded suspicions aimed at teenage swimming gold medalists Ye Shiwen of China and Katie Ledecky of the United States, for being so young and so fast.

The IOC's medical commission chairman Arne Lungqvist simply responded that it was a "sad day" when doubt was cast on each outstanding performance.

There likely will be sad days to follow at the London Olympics. But not yet.