7.1 Patient Preparation and Physiologic Effects
Key Takeaways
- Fasting for glucose and many lipid panels is commonly 8–12 hours with water allowed; never label a nonfasting patient as fasting.
- Cortisol and serum iron show diurnal variation and are typically higher in the morning; an afternoon draw is not the same timed specimen.
- After mastectomy, draw the opposite arm unless a documented exception exists; never apply a tourniquet to an arteriovenous fistula or graft arm.
- A therapeutic-drug-monitoring trough is drawn immediately before the next scheduled dose; an oral glucose tolerance test needs a fasting baseline, then timed draws after the glucose load.
- Prolonged tourniquet use, dehydration, and fist pumping cause hemoconcentration; anxiety can raise catecholamines and the white-cell count.
CMLA II.3.B.4–5 requires you to know physiological aspects of blood collection and to prepare the patient for various tests. Chapter 6 already covered identification, containers, and serum versus plasma. This section is the state of the patient before the needle: food, posture, time of day, exercise, smoking, hydration, intravenous (IV) fluids, restricted arms, and timed protocols. A technically perfect puncture on an unprepared patient is still a failed collection.
Quick Answer: Match the physiologic state to the order. Fasting is commonly 8–12 hours, not 24-hour starvation. Sitting versus supine changes proteins and lipids. Cortisol and iron are typically higher in the morning. Draw opposite a mastectomy; never tourniquet a fistula or graft arm. A trough is immediately before the next dose. Anxiety and a long tourniquet hemoconcentrate.
Fasting versus nonfasting
Fasting means no caloric intake for the interval in the test directory—commonly overnight 8–12 hours for glucose and many lipid panels—with water usually allowed. It is not a 24-hour fast and not a standing order for every chemistry tube. Nonfasting (random) specimens are valid when the menu says so; they are invalid when the order is labeled fasting and the patient just ate.
Ask what and when the patient last ate or drank other than water. Cream in coffee, juice, gum, candy, and alcohol break a fast. Smoking during a fast can alter glucose and catecholamines. If the order requires fasting and the patient is not fasting, do not relabel the tubes as fasting. Document last intake and follow standard operating procedure (SOP): recollect versus process as nonfasting.
Lipids and glucose are the classic fasting pair. Many electrolytes, complete blood counts (CBCs), and coagulation tests do not require a fast. Preparing the patient for various tests means reading the order, not applying one kitchen-table rule to every stopper color.
Posture: sitting versus supine
Posture moves plasma water. Compared with supine (lying) rest, sitting or standing lets fluid leave the vascular space, so proteins, enzymes, lipids, and blood cells concentrate (hemoconcentration). A cholesterol or albumin drawn after the patient has been upright is not the same number as one drawn after recumbency. Inpatients are often supine; outpatients are usually seated. That difference is a physiologic variable, not a reason to invent a secret percentage for CMLA.
Standardize: typically seat the patient, support the arm, and do not draw immediately after a brisk walk from the parking garage when the test is posture-sensitive. If policy requires a seated rest, follow it. Document unusual posture when the SOP says to. Do not stand a dizzy patient to make the vein pop.
Diurnal variation: cortisol and iron
Diurnal variation is a predictable 24-hour pattern.
| Analyte or situation | Typical pattern | What you do |
|---|---|---|
| Cortisol (and the related adrenocorticotropic hormone pattern) | Highest in the morning, lowest late evening | An AM cortisol at 16:00 is the wrong window |
| Serum iron | Often higher in the morning | Morning iron studies are not interchangeable with late-afternoon iron |
| Therapeutic drugs | Follow the dosing clock | Therapeutic drug monitoring is timed to the dose, not to sunrise |
| Many routine chemistries | Smaller time-of-day effects | Still label the actual collection time |
If the requisition specifies a timed hormone and the patient arrives at the wrong hour, do not collect and pretend the clock did not matter. Reschedule or document per SOP.
Exercise, smoking, dehydration, and IV fluids
Exercise. Recent strenuous activity raises muscle enzymes—creatine kinase (CK), aspartate aminotransferase (AST), lactate dehydrogenase (LDH)—plus potassium, lactate, and often the white-cell count. A baseline CK after a workout is not a resting baseline. Ask about recent heavy exercise when muscle enzymes or potassium are on the order.
Smoking. Acute smoking raises carboxyhemoglobin and catecholamines and can nudge glucose and white-cell counts. Many oral glucose tolerance test (OGTT) procedures forbid smoking during the test. Document when the directory cares.
Dehydration. Low plasma water hemoconcentrates hematocrit, proteins, urea nitrogen, and many chemistry values. Water is usually allowed during a fast; forcing fluids against a strict nothing-by-mouth order is not helpfulness.
IV fluids. Infusion dilutes the specimen if you collect from that arm, especially proximal to (above) the IV. Preferred site: the opposite arm. If that arm is unusable, follow Clinical and Laboratory Standards Institute (CLSI) PRE02 and facility SOP: stop the infusion when required, wait the specified interval, draw distal to (below) the IV, and discard the specified volume. A running IV is not a convenient extra vein.
Restricted arms: mastectomy and fistula or graft
After mastectomy or axillary lymph-node dissection, collect from the opposite arm. Tourniquet and puncture on the ipsilateral arm increase lymphedema and infection risk. Bilateral mastectomy, or an unusable opposite arm, needs a documented exception and the least-risk site the policy names. The old surgical arm looks fine is not an exception.
An arteriovenous (AV) fistula or graft is dialysis access. It is never a venipuncture site. Do not apply a tourniquet to that arm. Draw the opposite arm or another policy-approved site. A thrill under your fingers is not a median cubital vein.
Arms with a peripherally inserted central catheter (PICC) or other vascular access are likewise restricted unless SOP explicitly allows a specific workaround. When in doubt, stop and ask; do not improvise on the access arm.
Timed tests: TDM trough and OGTT
Therapeutic drug monitoring (TDM). A trough is the specimen drawn immediately before the next scheduled dose—the lowest expected concentration. Morning labs two hours after a dose is not a trough. Peak times are drug-specific; follow the order. Label date and time. Pharmacy cannot interpret an untimed drug level.
OGTT. Prepare the patient: fasting as ordered, then a fasting draw, then the measured glucose load, then timed specimens (commonly 1-hour and 2-hour in adult protocols—the exact minutes and load are the procedure, not a number to invent on CMLA). The patient remains, and does not eat, smoke, or leave for coffee. Vomiting the load or missing a time point usually means stop and notify. Do not label a late tube as the missed time.
Anxiety and hemoconcentration
Anxiety raises catecholamines, can increase white cells by demargination, and may precede syncope. A frightened patient who pumps a fist while you leave a tourniquet on is also creating hemoconcentration: plasma water leaves the occluded arm, concentrating cells, proteins, and potassium.
This section introduces the physiology. Section 7.2 limits tourniquet time to about one minute; Section 7.4 treats potassium and hemolysis as specimen-integrity failures. The mechanism starts here: prolonged occlusion plus muscle contraction is not a better vein. It is a chemically altered arm.
In practice
A seated outpatient needs a fasting lipid panel, an AM cortisol, and a vancomycin trough. You confirm an overnight fast with water only, that the clock matches a morning cortisol window, and that the trough is immediately before the next dose—not leftover morning-lab convenience. You ask about mastectomy, fistula, graft, and IV sites before you pick an arm. If the patient ate two hours ago, you do not write fasting on the label.
Exam traps
- Fasting is commonly 8–12 hours, not 24 hours, and is not required for every test.
- Sitting versus supine changes proteins, lipids, and cell counts.
- Cortisol and iron are the high-yield diurnal pair.
- Trough means before the next dose, not with AM labs.
- Opposite arm after mastectomy; no tourniquet on a fistula or graft arm.
- Draw the non-IV arm, or below a stopped IV—not above a running infusion.
- Anxiety and a long tourniquet hemoconcentrate; they are not harmless.
Capillary and infant collections, post-puncture care, send-outs, and legal-evidence specimens are Chapter 8. When CMLA names a test, answer with the physiologic variable that would make that specimen the wrong snapshot.
A patient has a left-arm arteriovenous fistula and a history of right-sided mastectomy. Which collection rule is correct?
A provider orders a vancomycin trough and an oral glucose tolerance test. Which timing statement is correct?
Which statement correctly describes a physiologic effect the collector must prevent or document?