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Klingon blood screening

A Klingon blood screening

A blood screening was a type of blood test that was initially popular as a means of determining if an individual was a Changeling. It was used through the logic that once blood was drawn from a subject, if they were a Changeling, their matter would revert to its gelatinous state.

Odo first suggested the procedure in late 2371 when a Changeling hijacked the USS Defiant. However, the tactic failed because the Changeling had replaced Julian Bashir, who performed the screenings, and was able to frame Michael Eddington as the Changeling instead. (DS9: "The Adversary")

Dukat blood screening

Dr. Bashir performing a blood screening on Gul Dukat

By early 2372, the Klingons also used blood screenings to detect Changelings, but their method was cruder – they simply used an available blade (such as a d'k tahg) to break the skin and allow some blood to escape. (DS9: "The Way of the Warrior") It was not discovered until much later that a Changeling was impersonating General Martok, and was able to evade the blood screenings. (DS9: "Apocalypse Rising")

Blood screening vial faked

Fake blood screening positive results

When the Changelings threatened Earth in mid-2372, Starfleet Command adopted an official policy, recommended by Benjamin Sisko (acting as temporary head of Starfleet Security on Earth) requiring weekly blood screenings of all Starfleet personnel and their immediate families. (DS9: "Homefront") However, Admiral Leyton managed to fake a blood screening to frame Sisko as a Changeling when Sisko was trying to expose Leyton's attempted coup against the Federation civilian government. (DS9: "Paradise Lost")

In a moment of anger, Joseph Sisko claimed that, were he a Changeling, the first thing he would do upon infiltrating Earth would be to kill someone and suck up all their blood, so he could use it to fool blood screenings. (DS9: "Homefront")

Despite their apparent ineffectiveness, blood screenings continued to be used on occasion as a stopgap measure to guard against Changeling infiltration. (DS9: "In Purgatory's Shadow", "By Inferno's Light")

It is notable, however, that despite the Federation's misplaced trust on blood screenings, blood screens were never successful: no Changeling was ever detected by a blood screening. In fact, from the start, Changelings such as the Krajensky Changeling and the Martok Changeling have used the Federation's reliance on blood screens to their own advantage.

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