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

In countries that screen all pregnant women (such as the US), a vaginal-rectal swab is taken at about 36–37 weeks. Which organism is it screening for?

  1. ACandida albicans
  2. BGroup B Streptococcus
  3. CHepatitis B virus
  4. DStaphylococcus aureus
2.Lab values & electrolytesHard

The nurse prepares a patient for a 2-hour oral glucose tolerance test tomorrow morning. Which statement shows a need for further teaching?

  1. A"I'll fast overnight but can drink plain water"
  2. B"I'll avoid smoking until the test is over"
  3. C"I can have a light breakfast before the test"
  4. D"I'll stay seated during the test if I can"
3.Arterial blood gas interpretationMedium
pH7.357.457.44PaCO₂mmHg354529HCO₃⁻mmol/L222619uncompensatedpartially compensatedfully compensated

A practice arterial blood gas shows pH 7.44, PaCO₂ 29 mmHg (3.9 kPa) and HCO₃⁻ 19 mmol/L. The primary disorder is respiratory alkalosis. How far is it compensated? Click your answer.

pH 7.44PaCO₂ 29 mmHg (3.9 kPa)HCO₃⁻ 19 mmol/L
  1. 1uncompensated
  2. 2partially compensated
  3. 3fully compensated
4.Arterial blood gas interpretationHard
pH7.357.457.20PaCO₂mmHg354548HCO₃⁻mmol/L222618appropriate compensationadded respiratory acidosisadded respiratory alkalosis

A practice gas shows a metabolic acidosis: pH 7.20, PaCO₂ 48 mmHg (6.4 kPa), HCO₃⁻ 18 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.

pH 7.20PaCO₂ 48 mmHg (6.4 kPa)HCO₃⁻ 18 mmol/L
  1. 1appropriate compensation
  2. 2added respiratory acidosis
  3. 3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
VCss​ = 10 mg/LR = ? mg/h · S = 0.8CL = 0.045 L/kg/h100 kg

Practice exercise (made-up values, not clinical guidance). A 100 kg patient is to receive an aminophylline infusion (salt factor S = 0.8: 80% of aminophylline is theophylline). The theophylline clearance is 0.045 L/kg/h. What aminophylline infusion rate R in mg/h gives a steady-state theophylline level of 10 mg/L?

100 kgCL = 0.045 L/kg/hCss = 10 mg/LS = 0.8
R = mg/h
6.Clinical pharmacokineticsHard
t (h)C (mg/L)Css​20 hC = ?

Practice exercise (made-up values, not clinical guidance). A vancomycin infusion runs at 60 mg/h with no loading dose. The patient's clearance is 3 L/h and the half-life is 8 h. What is the level C after 20 h?

R = 60 mg/hCL = 3 L/ht½ = 8 ht = 20 h
C = mg/L
7.Clinical nursingMedium

Which group of symptoms suggests hypercalcaemia?

  1. ATingling lips, muscle cramps and spasms of the hands and feet
  2. BSweating, tremor and sudden hunger
  3. CThirst, constipation, confusion and abdominal pain
  4. DWeight gain, feeling cold and a slow pulse
8.Clinical nursingHard

A patient with cirrhosis and a large tense abdomen undergoes paracentesis. Why may albumin be given?

  1. ATo stop the liver making more bile
  2. BTo treat infection in the fluid
  3. CTo increase urine potassium
  4. DTo reduce the risk of circulatory dysfunction after large-volume fluid removal

3Solutions

1.Lab values & electrolytes

Answer: B · Group B Streptococcus

Why this is the answer

Group B streptococci can colonize the vagina and rectum and can cause severe infection in newborns. Women who test positive receive IV antibiotics during labor to reduce transmission to the baby. The UK does not screen all women routinely; it uses a risk-based approach instead.Checked against: Group B streptococcal septicemia of the newborn · MedlinePlus (NIH) (2024)

Answer

B · Group B Streptococcus
2.Lab values & electrolytes

Answer: C · "I can have a light breakfast before the test"

Why this is the answer

The OGTT is done after an overnight fast of at least 8 hours; water is allowed. Eating before the test makes the results invalid. Patients rest during the test and avoid smoking, which can affect glucose.Checked against: Blood Glucose Test · MedlinePlus (NIH) (2024); Glucose tolerance test - non-pregnant · MedlinePlus (NIH) (2025)

Answer

C · "I can have a light breakfast before the test"
3.Arterial blood gas interpretation

Answer: fully 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 7.44: within 7.35–7.45
PaCO₂ 29 mmHg: low (35–45)
HCO₃⁻ 19 mmol/L: low (22–26)

Compensation

fully compensated

Answer

fully compensated
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

1.5·18 + 8 = 35 → 33–37 mmHg

Compare

measured: 48 mmHg → added respiratory acidosis

Answer

added respiratory acidosis
5.Clinical pharmacokinetics

Answer: R = 56.25 mg/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.

Clearance

CL = 0.045·100 = 4.5 L/h

Infusion rate

R = CL·Css/S = 4.5·10/0.8 = 56.25 mg/h

Answer

R = 56.25 mg/h
6.Clinical pharmacokinetics

Answer: C = 16.46 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.

Steady-state level

Css = 60/3 = 20 mg/L

Rate constant

k = 0.693/8 = 0.08664 h⁻¹

Level after the given time

C = 20·(1 − e−0.08664·20) = 16.46 mg/L

Answer

C = 16.46 mg/L
7.Clinical nursing

Answer: C · Thirst, constipation, confusion and abdominal pain

Why this is the answer

High calcium causes polyuria and thirst, constipation, abdominal pain, kidney stones and confusion. Tingling and cramps suggest low calcium instead.

Answer

C · Thirst, constipation, confusion and abdominal pain
8.Clinical nursing

Answer: D · To reduce the risk of circulatory dysfunction after large-volume fluid removal

Why this is the answer

Large-volume paracentesis can cause hypotension and renal impairment. Albumin infusion helps maintain the circulating volume.

Answer

D · To reduce the risk of circulatory dysfunction after large-volume fluid removal

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