**Core Concept**
During major surgery, accurate estimation of blood loss is crucial for patient management and transfusion requirements. The most reliable method of estimating blood loss involves direct measurement of the volume of blood lost, taking into account the suction canister and swab counts.

**Why the Correct Answer is Right**
The most accurate method of estimating blood loss is by weighing the swabs and suction canisters used during surgery. This method is more reliable than visual estimation or blood-soaked swab count, as it accounts for all the blood lost, including that absorbed by swabs and suction equipment. The weight of the swabs and suction canisters can be measured before and after use, providing a more accurate estimate of the blood loss.

**Why Each Wrong Option is Incorrect**
**Option A:** Visual estimation of blood loss is subjective and often inaccurate, as it relies on the surgeon's judgment and may not account for hidden blood loss.

**Option B:** Blood-soaked swab count is also an inaccurate method, as it does not account for the amount of blood absorbed by the swabs.

**Option C:** Indirect methods, such as using the patient's vital signs or laboratory values, are not reliable for estimating blood loss during surgery.

**Clinical Pearl / High-Yield Fact**
Always remember to weigh the swabs and suction canisters used during surgery to accurately estimate blood loss and guide transfusion requirements.

**Correct Answer:** D.