Lab Values for Nurses: cram pack

Formula sheets, the key method steps for every topic and 8 practice problems, with full solutions at the back.

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1Formulas and methods

Lab values & electrolytes

Method

  1. Compare the value with the laboratory’s own reference range, then with the patient’s baseline and trend.
  2. Check the sample: hemolysis, a delay, or blood drawn near an IV can give false results.
  3. Link the value to its typical signs and to the drugs and conditions that cause it.
  4. Look for interactions: low potassium or magnesium increases digoxin toxicity; dehydration raises lithium.
  5. Act on the patient: assess, hold a drug if the level is unsafe, read back critical values and inform the prescriber.

Arterial blood gas interpretation

Normal ranges
pH 7.35–7.45, PaCO₂ 35–45 mmHg (4.7–6.0 kPa), HCO₃⁻ 22–26 mmol/L
ROME
Respiratory: PaCO₂ opposite to pH; Metabolic: HCO₃⁻ equal to pH
Winter’s formula
PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2
Metabolic alkalosis
PaCO₂ ≈ 0.7·HCO₃⁻ + 21
Anion gap
AG = Na⁺ − (Cl⁻ + HCO₃⁻) ≈ 8–12
Albumin-corrected gap
AG + 2.5·(4.0 − alb g/dL) = AG + 0.25·(40 − alb g/L)
Delta ratio
(AG − 12)/(24 − HCO₃⁻)
P/F ratio
PaO₂/FiO₂
Alveolar gas equation
PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8
Serum osmolality
2·Na⁺ + glucose + urea (2·Na⁺ + glucose/18 + BUN/2.8)

Method

  1. Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
  2. Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
  3. Look at the other value: normal means uncompensated, abnormal in the compensating direction means partial compensation, and a normal pH with both abnormal means full compensation.
  4. In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
  5. Check a high anion gap with the delta ratio and, when albumin is low, with the corrected gap.

Clinical pharmacokinetics

Rate constant and half-life
k = ln 2/t½ = 0.693/t½
First-order decay
C = C₀·e−kt = C₀·(1/2)t/t½
Two levels
k = ln(C₁/C₂)/Δt
Volume and clearance
Vd = dose/C₀, CL = k·Vd
Loading dose
LD = Vd·C/(S·F)
Maintenance rate
R = CL·Css/(S·F)
Average steady state
Css,avg = S·F·D/(CL·τ)
Steady state
1 − (1/2)ⁿ after n half-lives; ≈ 97% after 5
Infusion
C = (R/CL)(1 − e−kt)
Interval and dose from peak and trough
τ = ln(Cpeak/Ctrough)/k, D = Vd·(Cpeak − Ctrough)
Cockcroft–Gault
CrCl = (140 − age) × kg/(72 × SCr mg/dL), × 0.85 for women
Renal adjustment
dose × CrCl/normal; Dettli Q = 1 − fe·(1 − CrCl/normal)

Method

  1. Convert the half-life to a rate constant: k = 0.693/t½.
  2. Turn per-kg values into totals: Vd (L) = L/kg × weight, CL (L/h) = L/kg/h × weight.
  3. Pick the relation: C = C₀·e−kt, LD = Vd·C/(S·F), maintenance rate = CL·Css/(S·F), τ = ln(Cpeak/Ctrough)/k.
  4. For kidney function, estimate CrCl with Cockcroft–Gault first, then scale the dose or the interval.
  5. Check the result: levels fall by half every half-life, and steady state takes about five half-lives.

2Practice problems

1.Lab values & electrolytesMedium

When should blood cultures be taken for a patient with suspected sepsis?

  1. AAt any time, since antibiotics do not affect the result
  2. BBefore the first antibiotic dose, without delaying it much
  3. COnly if the fever lasts more than three days
  4. DAfter the first three doses of antibiotics
2.Lab values & electrolytesHard

The nurse teaches a patient with iron-deficiency anemia who is starting oral iron. Which statement shows a need for further teaching?

  1. A"I'll keep the tablets out of reach of the children"
  2. B"I can take it with orange juice to help absorption"
  3. C"My stools may look darker while I take the tablets"
  4. D"I'll take my iron tablet with a big glass of milk"
3.Arterial blood gas interpretationMedium

Practice values: Na⁺ 136 mmol/L, glucose 21.9 mmol/L and urea 19.9 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.

Na⁺ 136 mmol/Lglucose 21.9 mmol/Lurea 19.9 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard

A practice gas shows a metabolic alkalosis with HCO₃⁻ 38 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.

HCO₃⁻ 38 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
110100t (h)C (mg/L)1 h10 mg/L5 h2.5 mg/L

Practice exercise (made-up values, not clinical guidance). After an IV dose of tobramycin, the level is 10 mg/L at 1 h and 2.5 mg/L at 5 h. Assume first-order elimination. Find the elimination rate constant k.

C1 = 10 mg/L (1 h)C2 = 2.5 mg/L (5 h)
k = h⁻¹
6.Clinical pharmacokineticsHard
VCss,avg​ = ?750 mg · q12hCL = 3 L/hS = 1 · F = 1

Practice exercise (made-up values, not clinical guidance). A patient receives vancomycin IV every 12 h; the vancomycin clearance is 3 L/h. Each dose is 750 mg of vancomycin. Find the average steady-state level Css,avg.

CL = 3 L/hτ = 12 hS = 1F = 1D = 750 mg
Css,avg = mg/L
7.Clinical nursingMedium

Which technique is correct when suctioning through a tracheostomy?

  1. AApply suction only while withdrawing the catheter, for no more than about 10–15 seconds
  2. BSuction every 15 minutes whether or not secretions are present
  3. CApply suction continuously while inserting the catheter as far as it will go
  4. DKeep suctioning for at least a minute each time to clear all the secretions
8.Clinical nursingHard

A patient with several days of vomiting and diarrhoea has low blood pressure, low urine output and a rising creatinine. Which description fits best?

  1. AChronic kidney disease, treated with long-term dialysis three times a week
  2. BIntrinsic kidney injury from glomerulonephritis, treated with high-dose steroids
  3. CPost-renal acute kidney injury from obstruction, treated with a catheter
  4. DPre-renal acute kidney injury from poor perfusion, treated by restoring fluid volume

3Solutions

1.Lab values & electrolytes

Answer: B · Before the first antibiotic dose, without delaying it much

Why this is the answer

Blood cultures are taken before antibiotics, because even one dose can prevent bacteria from growing and hide the cause. Antibiotics should still be started quickly, ideally within an hour.Checked against: Sepsis · MedlinePlus (NIH) (2024)

Answer

B · Before the first antibiotic dose, without delaying it much
2.Lab values & electrolytes

Answer: D · "I'll take my iron tablet with a big glass of milk"

Why this is the answer

Calcium in milk, as well as antacids and tea, reduces iron absorption, so iron should not be taken with them. Vitamin C improves absorption, dark stools are expected, and iron overdose is a serious poisoning risk in young children.Checked against: Iron deficiency anemia - Medical Encyclopedia · MedlinePlus (NIH) (2025); Iron in diet - Medical Encyclopedia · MedlinePlus (NIH) (2025)

Answer

D · "I'll take my iron tablet with a big glass of milk"
3.Arterial blood gas interpretation

Answer: Osm = 314 mOsm/kg

Derived values: calculated serum osmolality is 2·Na⁺ + glucose + urea with all values in mmol/L (2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL); normal is about 275–295 mOsm/kg. The P/F ratio is PaO₂ divided by FiO₂ as a fraction. The alveolar oxygen pressure is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 mmHg at sea level, and the A–a gradient is PAO₂ − PaO₂.

Calculated osmolality

Osm = 2·136 + 21.9 + 19.9 = 313.8 ≈ 314 mOsm/kg

Answer

Osm = 314 mOsm/kg
4.Arterial blood gas interpretation

Answer: PaCO₂ = 47.6 mmHg

Expected compensation: in metabolic acidosis Winter’s formula predicts PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2 mmHg; a measured value above that range points to an additional respiratory acidosis, below it to an additional respiratory alkalosis. The anion gap AG = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mmol/L, separates high-gap from normal-gap metabolic acidosis; low albumin hides a high gap (add 2.5 per g/dL that albumin is below 4.0). The delta ratio (AG − 12)/(24 − HCO₃⁻) between about 0.8 and 2 fits a pure high-gap acidosis (some texts use 1 instead of 0.8), below that an added normal-gap acidosis, and above 2 an added metabolic alkalosis.

Expected PaCO₂

0.7·38 + 21 = 47.6 mmHg

Answer

PaCO₂ = 47.6 mmHg

The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.

5.Clinical pharmacokinetics

Answer: k = 0.3466 h⁻¹

Clearance and volume: at steady state the dosing rate equals the elimination rate, so the maintenance rate = CL × Css. A loading dose is LD = Vd × C/(S × F), with S the salt factor (fraction of active drug in the salt) and F the bioavailability (1 for IV). From two levels, k = ln(C₁/C₂)/Δt; for an IV bolus Vd = dose/C₀ and CL = k × Vd. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

Elimination rate constant

k = ln(10/2.5)/4 = 1.386/4 = 0.3466 h⁻¹

Answer

k = 0.3466 h⁻¹
6.Clinical pharmacokinetics

Answer: Css,avg = 20.83 mg/L

Designing a regimen: with rapid IV doses the level falls from peak to trough during the interval, so τ = ln(Cpeak/Ctrough)/k and the dose that restores the peak is Vd × (Cpeak − Ctrough). The average steady-state level is Css,avg = S·F·dose/(CL·τ). An infusion rises as C = (R/CL)(1 − e−kt). For a drug cleared by the kidneys, the dose is scaled with the creatinine clearance. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

Average steady-state level

Css,avg = S·F·D/(CL·τ) = 1·1·750/(3·12) = 20.83 mg/L

Answer

Css,avg = 20.83 mg/L
7.Clinical nursing

Answer: A · Apply suction only while withdrawing the catheter, for no more than about 10–15 seconds

Why this is the answer

Suctioning only on withdrawal and keeping it brief reduces hypoxia and mucosal damage. Suction is performed when needed, not routinely, and the patient is pre-oxygenated if required.

Answer

A · Apply suction only while withdrawing the catheter, for no more than about 10–15 seconds
8.Clinical nursing

Answer: D · Pre-renal acute kidney injury from poor perfusion, treated by restoring fluid volume

Why this is the answer

Fluid loss reduces kidney perfusion, causing pre-renal AKI. Prompt fluid replacement can restore function before the kidney tissue itself is damaged.

Answer

D · Pre-renal acute kidney injury from poor perfusion, treated by restoring fluid volume

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