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 vancomycin trough level is above the target range. Which adverse effect is the main concern?

  1. AKidney damage (nephrotoxicity)
  2. BA raised platelet count (thrombocytosis)
  3. CLow blood glucose (hypoglycemia)
  4. DConstipation and bloating
2.Lab values & electrolytesHard

A patient with a large, fast-growing lymphoma will start chemotherapy tomorrow. Which plan best prevents tumor lysis syndrome?

  1. AFluid restriction and a high-potassium diet before the treatment
  2. BNo special measures, because the risk starts only after a week
  3. CCalcium supplements and a high-phosphate diet during treatment
  4. DIV hydration, a uric acid-lowering drug and frequent electrolytes
3.Arterial blood gas interpretationMedium
pH7.357.457.30PaCO₂mmHg354525HCO₃⁻mmol/L222612uncompensatedpartially compensatedfully compensated

A practice arterial blood gas shows pH 7.30, PaCO₂ 25 mmHg (3.3 kPa) and HCO₃⁻ 12 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.

pH 7.30PaCO₂ 25 mmHg (3.3 kPa)HCO₃⁻ 12 mmol/L
  1. 1uncompensated
  2. 2partially compensated
  3. 3fully compensated
4.Arterial blood gas interpretationHard
pH7.357.457.25PaCO₂mmHg354573HCO₃⁻mmol/L222631acutechronicacute on chronic

A practice gas shows a respiratory acidosis: pH 7.25, PaCO₂ 73 mmHg (9.7 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 7.25PaCO₂ 73 mmHg (9.7 kPa)HCO₃⁻ 31 mmol/L
  1. 1acute
  2. 2chronic
  3. 3acute on chronic
5.Clinical pharmacokineticsMedium
VCss​ = 4 mg/LR = ? mg/minCL = 500 mL/min

Practice exercise (made-up values, not clinical guidance). A patient has a lidocaine clearance of 500 mL/min. What infusion rate R in mg/min keeps the steady-state level at 4 mg/L?

CL = 500 mL/minCss = 4 mg/L
R = mg/min
6.Clinical pharmacokineticsHard
VCss,avg​ = ?450 mg · q12hCL = 0.05 L/kg/h64 kg · S = 1 · F = 1

Practice exercise (made-up values, not clinical guidance). A 64 kg patient takes oral theophylline (F = 1) every 12 h. The clearance is 0.05 L/kg/h. Each dose is 450 mg of theophylline. Find the average steady-state level Css,avg.

CL = 0.05 L/kg/hτ = 12 hS = 1F = 164 kgD = 450 mg
Css,avg = mg/L
7.Clinical nursingMedium

After a laparoscopic cholecystectomy, a patient has pain in the shoulder tip. What is the most likely cause?

  1. AA broken collarbone from being moved onto the table
  2. BAn allergic reaction to the dressing
  3. CGas used during the operation irritating the diaphragm
  4. DA heart attack caused by the stress of surgery
8.Clinical nursingHard

A patient with hyperkalaemia is given IV insulin with glucose. What is the purpose, and what must be monitored?

  1. ATo protect the heart muscle directly; the patient must be kept nil by mouth
  2. BTo treat hidden diabetes; HbA1c must be checked daily
  3. CTo remove potassium from the body; urine output must be stopped for a time
  4. DTo shift potassium into cells; blood glucose must be monitored for hypoglycaemia

3Solutions

1.Lab values & electrolytes

Answer: A · Kidney damage (nephrotoxicity)

Why this is the answer

High vancomycin levels are linked to acute kidney injury and, less often, hearing damage. The nurse reports the result before the next dose and checks the creatinine and urine output.

Answer

A · Kidney damage (nephrotoxicity)
2.Lab values & electrolytes

Answer: D · IV hydration, a uric acid-lowering drug and frequent electrolytes

Why this is the answer

Prevention includes aggressive IV hydration, drugs such as allopurinol or rasburicase to lower uric acid, and frequent monitoring of potassium, phosphate, calcium, uric acid and creatinine. Tumor lysis usually begins within 1–3 days of starting treatment.Checked against: Uric Acid Test · MedlinePlus (NIH) (2026)

Answer

D · IV hydration, a uric acid-lowering drug and frequent electrolytes
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 7.30: acidaemia (below 7.35)
PaCO₂ 25 mmHg: low (35–45)
HCO₃⁻ 12 mmol/L: low (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 + 1·3.3 = 27.3 mmol/L
chronic: 24 + 3.5·3.3 = 35.55 mmol/L

Compare

measured: 31 mmol/L → acute on chronic

Answer

acute on chronic
5.Clinical pharmacokinetics

Answer: R = 2 mg/min

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.

Clearance in L/min

500 mL/min = 0.5 L/min

Infusion rate

R = 0.5·4 = 2 mg/min

Answer

R = 2 mg/min
6.Clinical pharmacokinetics

Answer: Css,avg = 11.72 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.

Clearance

CL = 0.05·64 = 3.2 L/h

Average steady-state level

Css,avg = S·F·D/(CL·τ) = 1·1·450/(3.2·12) = 11.72 mg/L

Answer

Css,avg = 11.72 mg/L
7.Clinical nursing

Answer: C · Gas used during the operation irritating the diaphragm

Why this is the answer

Carbon dioxide used to inflate the abdomen can irritate the diaphragm, causing referred shoulder-tip pain. It usually settles within a few days with mobilisation and analgesia.

Answer

C · Gas used during the operation irritating the diaphragm
8.Clinical nursing

Answer: D · To shift potassium into cells; blood glucose must be monitored for hypoglycaemia

Why this is the answer

Insulin drives potassium into cells, temporarily lowering serum levels. Glucose is given to prevent hypoglycaemia, but glucose monitoring is still needed for several hours.

Answer

D · To shift potassium into cells; blood glucose must be monitored for hypoglycaemia

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