NCCT Capillary and Special Collections 2 — Questions and Answers
Question 1: Which finger is generally preferred for capillary puncture in adults?
- Thumb
- Index finger
- Middle or ring finger (Correct answer)
- Pinky finger
Correct answer: Middle or ring finger
The middle and ring fingers are preferred because they have adequate blood flow and fewer nerve endings compared to the index finger or thumb.
The middle (third) and ring (fourth) fingers are the standard choice for adult capillary puncture. The thumb has a pulse and calluses, the index finger is highly sensitive and used for gripping, and the pinky finger has too little tissue. The middle and ring fingers provide an optimal balance of tissue depth, blood flow, and patient comfort.
Question 2: What is the maximum recommended depth for a lancet puncture on an infant's heel?
- 1.0 mm
- 2.0 mm (Correct answer)
- 3.0 mm
- 4.0 mm
Correct answer: 2.0 mm
The maximum safe depth for infant heel puncture is 2.0 mm to avoid reaching the calcaneus bone and causing osteomyelitis.
CLSI guidelines specify that heel puncture depth should not exceed 2.0 mm in neonates and infants. The distance from the skin surface to the calcaneus bone can be as little as 2.0 mm in premature infants. Puncturing too deeply can damage the bone, potentially causing osteomyelitis (bone infection), which is a serious complication.
Question 3: Which of the following specimens is collected using a cold agglutinin test procedure?
- Blood culture specimen
- Glucose tolerance specimen
- Specimen kept at 37 degrees C during transport (Correct answer)
- Arterial blood gas specimen
Correct answer: Specimen kept at 37 degrees C during transport
Cold agglutinin specimens must be kept warm at 37 degrees C during collection and transport to prevent cold-reacting antibodies from attaching to red blood cells at lower temperatures.
Cold agglutinins are antibodies that bind to red blood cells at temperatures below body temperature, causing them to clump. If the specimen cools, the antibodies activate and the test result will be falsely elevated. The tube must be prewarmed to 37 degrees C and transported immediately in a heat block or warm water to maintain temperature until the serum can be separated in the lab.
Question 4: When performing a capillary blood gas collection, which of the following is required?
- Using a tourniquet on the finger
- Warming the site prior to collection (Correct answer)
- Collecting into a plain red-top tube
- Applying a cold pack to the site
Correct answer: Warming the site prior to collection
Warming the capillary puncture site increases arterial blood flow to the area, making the capillary blood more closely reflect arterial values.
Capillary blood gas samples must approximate arterial blood composition. Warming the puncture site to 42 degrees C for 3-5 minutes with a warm towel or commercial heel warmer increases local blood flow by up to seven times and arterializes the capillary bed. Without warming, the specimen would contain more venous blood, yielding lower pO2 and higher pCO2 values that do not accurately reflect the patient's respiratory status.
Question 5: What is the correct procedure when the first drop of blood appears after a capillary puncture?
- Collect it immediately into the tube
- Wipe it away with sterile gauze (Correct answer)
- Blow on it to speed collection
- Apply pressure to increase flow
Correct answer: Wipe it away with sterile gauze
The first drop of blood is wiped away because it contains tissue fluid that can dilute the sample and affect test results.
When a capillary puncture is performed, the lancet cuts through tissue, releasing interstitial (tissue) fluid along with blood. This first drop is a mixture of blood and tissue fluid, which can dilute analytes and falsely lower results for tests like glucose or hemoglobin. Wiping it away with sterile gauze ensures that subsequent drops are primarily blood, providing more accurate laboratory results.
Question 6: During a glucose tolerance test (GTT), what must the patient do between blood draws?
- Eat a light snack every hour
- Remain fasting and avoid strenuous activity (Correct answer)
- Drink water with each blood draw
- Walk briskly for 10 minutes per hour
Correct answer: Remain fasting and avoid strenuous activity
Patients must remain fasting (except for water) and avoid strenuous activity during a GTT because eating or exercise can affect glucose metabolism and skew results.
A glucose tolerance test measures how efficiently the body processes a standardized glucose load over 2-3 hours. If the patient eats, additional glucose enters the bloodstream, invalidating the test. Strenuous exercise increases glucose uptake by muscles, potentially lowering blood glucose levels artificially. The patient should sit quietly, may drink water, and must not smoke, chew gum, or consume anything else during the testing period.
NCCT Capillary and Special Collections 2
Which finger is generally preferred for capillary puncture in adults?