Question

A patient with congestive heart failure has been successfully treated with digoxin for several years. Laboratory records indicate semiannual peak digoxin levels have all been in the therapeutic range. This patient recently developed renal failure, and admission testing was performed. Selected serum or blood laboratory results from this specimen are shown in Case Study Table 30-1.1. Although serum digoxin is high, the physician indicates the patient is not exhibiting signs or symptoms of toxicity.

1. If these results were derived from a random specimen, how may the time since the last dose affect the interpretation of the digoxin results?

2. Other than time, what additional factors should be taken into consideration when interpreting the digoxin results?

3. What additional laboratory test would aid in the interpretation of this case?

TABLE 30.1 Laboratory results below

Test                                              Result                                               Reference Range

Sodium                                         129                                                   135–145 mmol/L

Potassium                                   5.5                                                     3.5–5 mmol/L

Chloride                                      113                                                      97–107 mmol/L

Blood pH                                    7.25                                                     7.35–7.45

tCO2                                          16                                                         21–31 mmol/L

Urea nitrogen                          180                                                        5–20 mg/dL

Creatinine                                4.5                                                         0.6–1 mg/dL

Osmolality                               275                                                        282–300 mOsm/kg

Digoxin                                   2.5                                                          0.9–2 ng/mL

Answer 1