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 a heparin infusion has an aPTT of 110 seconds; the protocol target is 1.5–2.5 times the control of 30 seconds. What should the nurse do?

  1. AGive vitamin K as prescribed and continue the infusion
  2. BFollow the heparin protocol, check for bleeding and inform the provider
  3. CIncrease the infusion rate, because the result is above the target range
  4. DContinue unchanged and repeat the aPTT the next morning
2.Lab values & electrolytesHard

A patient's total calcium is 7.6 mg/dL and the albumin is 2.0 g/dL. Using the common correction (add 0.8 mg/dL for each 1 g/dL that albumin is below 4.0), what is the corrected calcium?

  1. A6.0 mg/dL, well below the normal range
  2. B7.6 mg/dL, below the normal range
  3. C9.2 mg/dL, within the normal range
  4. D11.6 mg/dL, above the normal range
3.Arterial blood gas interpretationMedium

A practice gas shows a metabolic acidosis with HCO₃⁻ 18 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.

HCO₃⁻ 18 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
pH7.357.457.13PaCO₂mmHg354528HCO₃⁻mmol/L22269appropriate compensationadded respiratory acidosisadded respiratory alkalosis

A practice gas shows a metabolic acidosis: pH 7.13, PaCO₂ 28 mmHg (3.7 kPa), HCO₃⁻ 9 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.

pH 7.13PaCO₂ 28 mmHg (3.7 kPa)HCO₃⁻ 9 mmol/L
  1. 1appropriate compensation
  2. 2added respiratory acidosis
  3. 3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
110t (h)C (mg/L)C₀ = 8 mg/Lt½ = 3 hC₀/2

Practice exercise (made-up values, not clinical guidance). A patient receives an IV bolus of 200 mg of gentamicin. The level back-extrapolated to time zero is 8 mg/L and the half-life is 3 h. Find the volume of distribution Vd.

D = 200 mgC₀ = 8 mg/Lt½ = 3 h
Vd = L
6.Clinical pharmacokineticsHard
Vd = 0.3 L/kg0.5 → 6 mg/LD = ? mg61 kg

Practice exercise (made-up values, not clinical guidance). A 61 kg patient on regular tobramycin doses is at steady state. The target is a peak of 6 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?

61 kgVd = 0.3 L/kgCpeak = 6 mg/LCtrough = 0.5 mg/L
D = mg
7.Clinical nursingMedium

A patient's NEWS2 score has risen to 7. What is the appropriate response?

  1. AGive a sedative and recheck later
  2. BEmergency assessment by a clinical team with critical care skills
  3. CContinue observations every 12 hours
  4. DRecord the score, increase observations and wait for the morning ward round
8.Clinical nursingHard

A patient with stress incontinence is taught self-care. Which statement shows a need for further teaching?

  1. A"I'll cut down on caffeine."
  2. B"I'll try to lose some weight if I can."
  3. C"I'll do my pelvic floor exercises every day."
  4. D"I'll drink as little as possible so I don't leak."

3Solutions

1.Lab values & electrolytes

Answer: B · Follow the heparin protocol, check for bleeding and inform the provider

Why this is the answer

110 s is well above the target of about 45–75 s, so the bleeding risk is high. The nurse follows the heparin protocol, assesses for bleeding and informs the provider. Protamine is the reversal agent for heparin; vitamin K reverses warfarin.Checked against: Partial Thromboplastin Time (PTT) Test · MedlinePlus (NIH) (2026)

Answer

B · Follow the heparin protocol, check for bleeding and inform the provider
2.Lab values & electrolytes

Answer: C · 9.2 mg/dL, within the normal range

Why this is the answer

Corrected calcium = 7.6 + 0.8 × (4.0 − 2.0) = 9.2 mg/dL, which is normal. Low albumin lowers the bound fraction of calcium; the correction is only an estimate, and an ionized calcium is more accurate when it matters.Checked against: Calcium blood test · MedlinePlus (NIH) (2025)

Answer

C · 9.2 mg/dL, within the normal range
3.Arterial blood gas interpretation

Answer: PaCO₂ = 35 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.

Winter’s formula

PaCO₂ = 1.5·18 + 8 = 35 mmHg
range: 33–37 mmHg

Answer

PaCO₂ = 35 mmHg
4.Arterial blood gas interpretation

Answer: added respiratory acidosis

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

1.5·9 + 8 = 21.5 → 19.5–23.5 mmHg

Compare

measured: 28 mmHg → added respiratory acidosis

Answer

added respiratory acidosis
5.Clinical pharmacokinetics

Answer: Vd = 25 L

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.

Volume of distribution

Vd = 200/8 = 25 L

Answer

Vd = 25 L
6.Clinical pharmacokinetics

Answer: D = 100.7 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 = 0.3·61 = 18.3 L

Dose

D = 18.3·(6 − 0.5) = 100.7 mg

Answer

D = 100.7 mg

In practice the dose is rounded to a convenient amount.

7.Clinical nursing

Answer: B · Emergency assessment by a clinical team with critical care skills

Why this is the answer

In NEWS2, a score of 7 or more triggers an emergency response, usually continuous monitoring and urgent review by a team competent in managing critically ill patients.

Answer

B · Emergency assessment by a clinical team with critical care skills
8.Clinical nursing

Answer: D · "I'll drink as little as possible so I don't leak."

Why this is the answer

Restricting fluids concentrates urine, irritates the bladder and risks dehydration and infection. Pelvic floor exercises, weight loss and reducing caffeine are recommended measures.

Answer

D · "I'll drink as little as possible so I don't leak."

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