Special Blood Collections: Timed Draws, Cultures, and Chain of Custody

A routine draw needs a good vein and a correct label. Special collections need more, and each one has a rule that decides if the result can be used. A drug level drawn 40 minutes late can lead to a wrong dose. A blood culture with skin bacteria can put a patient on antibiotics they do not need. A legal sample with a gap in its paperwork may be thrown out. Before any special collection, read the order and your lab's test directory. They set the rules for your site, and they change more often than textbooks do.

What You Will Be Able to Do

• Collect timed drug levels and record them correctly.
• Run a glucose tolerance test on schedule and know when to stop it.
• Collect blood cultures with the right volume and low contamination.
• Handle a legal sample without breaking the chain of custody.
• Change your order of draw for capillary collection.

Before You Go: Read the Directory

Stop at the order first. It takes 30 seconds, and it can save a second stick.

1. Test name. Is it the exact test the provider wants?
2. Tube. Does it need a tube you do not usually carry?
3. Timing. Is there a time window, or a link to a dose or a drink?
4. Patient prep. Fasting, a drug hold or a set diet?
5. Handling. Ice, warmth, darkness or a time limit to reach the lab?
6. Paperwork. Any special form, witness or consent?

If any line is unclear, call the lab before you collect.

Timed Drug Levels

Some medicines work only in a narrow range. Too little does not treat the problem, and too much can damage the kidneys or hearing. Providers and pharmacists check blood levels and then adjust the dose, so the level only makes sense if everyone knows how long it has been since the last dose.

• Trough: the lowest level, drawn just before the next dose.
• Peak: the highest level, drawn a set time after a dose.

The window comes from the order or the pharmacy, and it varies by drug. Do not assume.

The Steps

1. Check the order for the drug name and the collection time.
2. Ask the nurse when the last dose was given and when the next dose is due.
3. Draw from the arm without the infusion. A sample from a site near the infusion can read falsely high.
4. Write the real collection time on the label, not the time on the order.
5. Get the sample to the lab.

Worked example. The order says: vancomycin trough, draw before the 09:00 dose. You reach the room at 09:12, and the nurse has already started the infusion. This is no longer a trough, so stop. Tell the nurse and let them decide with the pharmacy. They may ask for the level to be timed around the next dose instead. If you drew it anyway and wrote "08:55," the pharmacist could cut a dose based on a false number.

Warning: Never change a collection time to match the order. If you are late, write the real time and tell the nurse. A late level with the right time can often still be used. A wrong time cannot be fixed later.

Glucose Tolerance Testing

An oral glucose tolerance test (OGTT) measures how the body handles a sugar load. The standard adult test for diabetes uses a drink with 75 grams of glucose. Testing during pregnancy can use other doses and schedules, so follow the order.

1. Check the fast. The patient should have had no food or drinks except water for at least 8 hours. Some labs set an upper limit too, such as 14 hours. Ask, and record it.
2. Draw the fasting sample first. Many labs check this result before giving the drink. If the fasting glucose is high, the lab or provider may cancel the rest of the test.
3. Give the drink as your lab directs. It usually needs to be finished within about 5 minutes.
4. Start the clock when the patient finishes the drink.
5. Draw each sample on time, and label each with the exact time.
6. Keep the patient resting. No food, gum or smoking until the test ends. Walking around can change the result.

Worked example. A 2-hour OGTT. The fasting draw is at 07:50. The patient finishes the drink at 08:04. The 1-hour draw is due at 09:04, and the 2-hour draw at 10:04, if the order calls for both.

Stop the test and tell the nurse or lab if the patient vomits, because the sugar load is no longer known. Also stop if they become shaky, sweaty, pale, confused or faint. Have them sit or lie down. These can be signs of low blood sugar, and you need help.

Blood Cultures

Blood cultures look for bacteria or yeast in the blood. They are the main test for a bloodstream infection, which can be life-threatening. They also go wrong more easily than other tests, because skin bacteria are always close by.

CDC's quality tool for adult blood cultures gives clear numbers:

• Volume: 20 to 30 mL per set by venipuncture, which is 10 mL per bottle in a two-bottle set.
• Sets: usually two, from separate sites, so 40 mL in total.
• Contamination: a well-run practice should stay well under 3 percent.

Volume matters because bacteria can be few and far between. A half-filled bottle can miss a real infection, so aim for the fill mark and do not overfill. If you cannot get enough blood, follow your procedure for which bottle to fill first, and tell the lab.

Keeping Skin Bacteria Out

1. Clean your hands and put on gloves.
2. Find the vein before you clean. Once the site is clean, do not touch it again, even with a gloved finger.
3. Clean the site with the antiseptic your procedure names, usually chlorhexidine, and let it dry fully. The drying time is part of how it works.
4. Clean the rubber top of each bottle with 70 percent isopropyl alcohol, and let it dry. A cap on a new bottle does not mean the top is sterile.
5. With a butterfly, fill the aerobic bottle first, so that air in the tubing does not reach the anaerobic bottle. Follow your procedure for a syringe draw.
6. Mark the fill level if your lab asks, and label each bottle with the site, such as "left arm."
7. Fill the cultures first, before any other tubes.

Cultures should be collected by venipuncture. Drawing from an existing line raises the contamination risk, so a nurse does this only when it has been ordered.

Worked example. Your lab counted 540 culture sets last month, and 12 grew skin bacteria that were judged contaminants. 12 / 540 = 0.022, or 2.2 percent. This meets the under 3 percent measure, but the lab may still want to know which unit or collector the 12 came from. If your number climbs, check your drying time first.

Temperature and Light

A tube that needs special handling must start that handling at once.

Do not rest a tube on top of ice cubes, because it will not chill evenly. Warm samples need a heat block or a warm carrier, not your pocket.

Legal Samples and Chain of Custody

Chain of custody is the signed record of every person who handled a sample, from collection to testing. It proves that the sample tested is the sample that was taken and that no one could have tampered with it. Common legal samples are blood alcohol draws requested by police and some drug tests. Rules vary by state and by employer, so use your site's kit and form exactly.

For a blood alcohol draw:

1. Confirm your authority. Your facility's policy says who may draw and what paperwork is needed. If you are unsure, call your supervisor. Do not take direction only from an officer at the door.
2. Identify the patient. Use photo ID or the method your form requires, and record who witnessed it.
3. Do not use alcohol to clean the skin. Use a non-alcohol antiseptic such as povidone-iodine, or soap and water, as your kit directs. Alcohol residue can raise the result.
4. Use the tubes in the kit. These are usually gray tops, which hold a preservative. Invert them as directed.
5. Label and seal in front of the witness. Put on tamper-evident tape and sign where the form says.
6. Complete the form. Each person who takes the sample signs with the date and time.
7. Never leave the sample unattended. Keep it with you or locked up until the next person signs.

A broken seal or a missing signature can make the sample useless, and a sample cannot be repaired afterward. If a step goes wrong, tell your supervisor and write down what happened.

Capillary Collections

Capillary blood comes from a skin puncture on a finger or heel. It is a mix of blood from small vessels and tissue fluid, so the rules change.

The order changes. CLSI's capillary standard, GP42, uses a different order from venipuncture, because capillary blood can start to clot within seconds.

1. Blood gases, if ordered and within your role
2. EDTA tubes, such as lavender microtubes
3. Other additive tubes
4. Tubes with no additive
5. Filter paper, such as newborn screening cards

Do not use the venous order here. If you collect serum first, the EDTA tube may clot before you reach it.

Where to puncture

• Adults and older children: the side of the fingertip pad of the middle or ring finger. Avoid the thumb.
• Infants under 6 months: the heel only, on the outer or inner edge of the bottom of the foot. Never the back of the heel.
• Depth: heel punctures stay at 2.0 mm or less, so that the lancet does not reach bone. Use a device made for the patient's size.

Getting a good sample

1. Warm the site for a few minutes if it is cold. Warmers for this use stay at or below 42°C.
2. Clean the site and let it dry.
3. Puncture, and wipe away the first drop, which may hold tissue fluid or skin debris. Some bedside test devices use the first drop, so follow the device's directions.
4. Use gentle, intermittent pressure, and release between drops. Do not squeeze hard. This pushes tissue fluid into the sample and breaks red cells.
5. Mix each tube as it is filled.

Newborn screening cards have their own state rules. In general, fill each printed circle with one even drop from one side, do not touch the circles, and let the card dry flat away from heat. Your state program's instructions come first.

Common Mistakes

Check Your Understanding

1. You arrive to draw a gentamicin peak and the nurse says the infusion ended 20 minutes later than planned. What do you do?

Do not simply draw at the ordered time. Tell the nurse the time on the order, and ask them or the pharmacy to confirm the new draw time. Record the real collection time.

2. A patient in a glucose tolerance test vomits 25 minutes after the drink. What do you do?

Stop the test, tell the nurse or lab, and record what happened and when. The sugar load is no longer known.

3. You have cleaned the site for a blood culture, and you want to check the vein once more with your finger. What should you do instead?

Do not touch it. If you must feel the vein again, clean the site again and let it dry.

4. An officer asks you to do a blood alcohol draw, and you pick up your usual alcohol pad. What is wrong, and what else do you check?

Do not use alcohol on the skin. Use the non-alcohol antiseptic in the kit. Also confirm your facility's rules for legal draws, use the right tubes and complete the chain of custody form.

5. You need a lavender microtube and a serum microtube from a fingerstick. Which goes first, and why?

Lavender goes first. Capillary blood clots fast, so the EDTA tube is filled before the serum tube.

Key Takeaways

• Read the order and the test directory before you collect anything special.
• A timed level is only useful with a true collection time. Never adjust the time on paper.
• Time a glucose tolerance test from the moment the drink is finished, and stop if the patient vomits.
• Blood cultures need the right volume, a clean dry site, and a clean bottle top.
• A legal sample depends on an unbroken signed record and an alcohol-free site.
• Capillary order of draw is different. EDTA comes before serum.