Damage threshold

Damage Threshold is a combat statistic in Fallout, Fallout 2, and Fallout Tactics: Brotherhood of Steel. It was removed in Fallout 3, leaving Damage resistance as the only armor statistic. However, it was restored in Fallout: New Vegas.

Fallout, Fallout 2, Fallout Tactics
Damage Threshold (DT) in Fallout 1, 2, and Tactics is a flat reduction of incoming damage with a minimum of 0, so for example if the attacker stabs the target with a knife and deals 6 damage, as long as the target has 6+ DT they will take 0 damage.

Fallout: New Vegas
Damage Threshold (DT) is a derived static value that is directly subtracted from the damage of each shot taken. In simple terms if a shot is fired with a damage value of 20 and the target's DT value is 15, then 15 points of damage will be removed from the shot's damage. DT plays an integral part in combination with the target's Damage Resistance (DR) value to determine the amount of final damage inflicted upon the target.

The combined statistics of DT and DR (with DR being applied first) determines the total damage reduction applied to each shot, however it should be noted that it is not a simple issue of just subtracting the defined value from the damage of each shot, damage negated by DT will never go below 20% of the weapons's form damage.
 * $$\text{MaxOf}((\text{Attack Damage} - \text{DT}), (\text{Attack Damage} * 0.2)) =\text{Damage Inflicted}$$

It should be noted that if the target of an attack has 80+ DR, DT will have no effect as the game will not allow the damage modified by DT to go below 20% of the weapon's damage.

Damage which is reduced to below minimum value is indicated with a shield icon next to the health bar (either the PC, or the non-player character's). When all of an enemy's DT (if it's not 0 to begin with) is overcome with armor piercing attacks, then a broken shield icon will be appear in its place instead. The shield icon is shown during V.A.T.S. and on the HUD's health bars once damage has been inflicted upon a target and will remain for 3 seconds afterwards.

The default value for DT is set at 0, this is then modified mainly by the DT value of Armor and Clothing worn. Other attributes that can also effect the DT value both permanently or temporary of a character are Perks, Traits, Consumables and Ammunition.

The maximum DT achievable in Fallout: New Vegas is 95.5. This is can be done by taking both ranks of the Toughness (+6), Sub-Dermal Armor (+4), Reinforced Spine (+2), the third rank of Camarader-E (+2), active Nerd Rage (+15), Big Brained (character DT *1.1); having consumed Datura Hide (+1), Nuka-Cola Quartz (+6) and Whiskey with the Whiskey Rose perk (+9). The preceeding totals a base of 49.5, which can further be increased by wearing the Remnants Power Armor (+28) and helmet (+8), or a combination of an Atomic-valence tri-radii-oscillator, Dr.Klein or Mobius' glasses, and Ulysess' mask, while equipping the unique fatman Esther (+10), you reach a maximum of 95.5.

Bugs

 * In Fallout: New Vegas, the only articles of clothing that provide DR are the Rebreather, which is acquired during the quest Volare!, the Vault 11 jumpsuit, and the scientist outfit.
 * If the DT bar on your Pip-Boy is crossfading your amount of DR, then removing the DR while the DT number is not displayed as brightly as possible will result in the bar being completely blank until DR is reintroduced.