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

A patient on peritoneal dialysis notices that the drained fluid is cloudy and has abdominal pain. What should the nurse do?

  1. AStop dialysis for a week and recheck the fluid later
  2. BRecord it as normal and continue the usual exchanges
  3. CSend the fluid for cell count and culture and report it
  4. DAdd more glucose to the next bag to clear the fluid
2.Lab values & electrolytesHard

A patient took a large overdose of acetaminophen (paracetamol) 24 hours ago. Which laboratory changes are most expected?

  1. AA low ALT and AST and a low INR
  2. BA very high ALT and AST and a rising INR
  3. CA low white count with normal liver tests
  4. DA high troponin with a normal ALT
3.Arterial blood gas interpretationMedium
pH7.357.457.51PaCO₂mmHg354547HCO₃⁻mmol/L222636uncompensatedpartially compensatedfully compensated

A practice arterial blood gas shows pH 7.51, PaCO₂ 47 mmHg (6.3 kPa) and HCO₃⁻ 36 mmol/L. The primary disorder is metabolic alkalosis. How far is it compensated? Click your answer.

pH 7.51PaCO₂ 47 mmHg (6.3 kPa)HCO₃⁻ 36 mmol/L
  1. 1uncompensated
  2. 2partially compensated
  3. 3fully compensated
4.Arterial blood gas interpretationHard
pH7.357.457.26PaCO₂mmHg354572HCO₃⁻mmol/L222631acutechronicacute on chronic

A practice gas shows a respiratory acidosis: pH 7.26, PaCO₂ 72 mmHg (9.6 kPa), HCO₃⁻ 31 mmol/L. Use the rules ‘HCO₃⁻ rises 1 mmol/L per 10 mmHg (acute) or 3.5 mmol/L per 10 mmHg (chronic)’ to classify it. Click your answer.

pH 7.26PaCO₂ 72 mmHg (9.6 kPa)HCO₃⁻ 31 mmol/L
  1. 1acute
  2. 2chronic
  3. 3acute on chronic
5.Clinical pharmacokineticsMedium
110100t (h)C (mg/L)1 h25 mg/L17 h7.3 mg/L

Practice exercise (made-up values, not clinical guidance). After an IV dose of vancomycin, the level is 25 mg/L at 1 h and 7.3 mg/L at 17 h. Assume first-order elimination. Find the half-life t½.

C1 = 25 mg/L (1 h)C2 = 7.3 mg/L (17 h)
t½ = h
6.Clinical pharmacokineticsHard
Vd = 0.3 L/kg0.5 → 7 mg/LD = ? mg60 kg

Practice exercise (made-up values, not clinical guidance). A 60 kg patient on regular tobramycin doses is at steady state. The target is a peak of 7 mg/L and a trough of 0.5 mg/L, and the volume of distribution is 0.3 L/kg. Assuming rapid IV doses, what dose D raises the level from the trough to the peak?

60 kgVd = 0.3 L/kgCpeak = 7 mg/LCtrough = 0.5 mg/L
D = mg
7.Clinical nursingMedium

A patient with leukaemia has a very low platelet count. Which nursing precaution is appropriate?

  1. AGive medicines by intramuscular injection
  2. BTake temperatures rectally
  3. CEncourage contact sports to keep fit
  4. DUse a soft toothbrush and an electric razor
8.Clinical nursingHard

A nurse teaches a patient with a new ileostomy. Which statement shows a need for further teaching?

  1. A"My output will be firm and formed like before."
  2. B"I will drink extra fluids when it is hot."
  3. C"My stoma should look pink or red and moist."
  4. D"I will empty the bag when it is about a third to half full."

3Solutions

1.Lab values & electrolytes

Answer: C · Send the fluid for cell count and culture and report it

Why this is the answer

Cloudy effluent with abdominal pain suggests peritonitis, the main serious complication of peritoneal dialysis. A sample is sent for white cell count, Gram stain and culture, and antibiotics (often given in the dialysis fluid) are started as prescribed.Checked against: Dialysis - peritoneal · MedlinePlus (NIH) (2024)

Answer

C · Send the fluid for cell count and culture and report it
2.Lab values & electrolytes

Answer: B · A very high ALT and AST and a rising INR

Why this is the answer

A toxic dose damages liver cells, so ALT and AST rise steeply, and liver failure raises the INR. Acetylcysteine is the antidote and works best when given early.Checked against: Acetaminophen Toxicity · StatPearls (NCBI Bookshelf) (2026)

Answer

B · A very high ALT and AST and a rising INR
3.Arterial blood gas interpretation

Answer: partially compensated

Reading a blood gas (ROME): first the pH: below 7.35 is acidaemia, above 7.45 alkalaemia. Then find the value that explains it. PaCO₂ (normal 35–45 mmHg, 4.7–6.0 kPa) is the respiratory part and moves Opposite to the pH in a respiratory disorder; HCO₃⁻ (normal 22–26 mmol/L) is the metabolic part and moves in the same direction as the pH (Respiratory Opposite, Metabolic Equal). The other value shows the compensation: normal means uncompensated; abnormal in the direction that pulls the pH back while the pH is still abnormal means partially compensated; a normal pH with both values abnormal means fully compensated. Ranges differ slightly between laboratories; these are practice values.

ROME, value by value

pH 7.51: alkalaemia (above 7.45)
PaCO₂ 47 mmHg: high (35–45)
HCO₃⁻ 36 mmol/L: high (22–26)

Compensation

partially compensated

Answer

partially compensated
4.Arterial blood gas interpretation

Answer: acute on chronic

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 HCO₃⁻

acute: 24 + 1·3.2 = 27.2 mmol/L
chronic: 24 + 3.5·3.2 = 35.2 mmol/L

Compare

measured: 31 mmol/L → acute on chronic

Answer

acute on chronic
5.Clinical pharmacokinetics

Answer: t½ = 9.009 h

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(25/7.3)/16 = 1.231/16 = 0.07694 h⁻¹

Half-life

t½ = 0.693/0.07694 = 9.009 h

Answer

t½ = 9.009 h
6.Clinical pharmacokinetics

Answer: D = 117 mg

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.

Volume of distribution

Vd = 0.3·60 = 18 L

Dose

D = 18·(7 − 0.5) = 117 mg

Answer

D = 117 mg

In practice the dose is rounded to a convenient amount.

7.Clinical nursing

Answer: D · Use a soft toothbrush and an electric razor

Why this is the answer

Thrombocytopenia increases bleeding risk. Gentle oral care, electric razors and avoiding IM injections and rectal procedures help prevent bleeding.

Answer

D · Use a soft toothbrush and an electric razor
8.Clinical nursing

Answer: A · "My output will be firm and formed like before."

Why this is the answer

Ileostomy output is liquid to porridge-like because it bypasses the colon. The other statements describe correct care: good hydration, emptying the bag before it is too heavy and recognising a healthy stoma.

Answer

A · "My output will be firm and formed like before."

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