7.2 Point-of-Care, Laboratory & Imaging Tests

Domain II.C asks the CPNP-PC to order and interpret point-of-care (POC) tests, laboratory studies, and imaging that fit the hypothesis — and to know when the correct action is not to test. PNCB’s published FAQ is explicit: laboratory reference ranges are generally not provided on the exam. You are expected to know pediatric principles (hemoglobin changes with age; alkaline phosphatase runs higher in growing children) rather than reciting an adult lab card.

Point-of-care tests used in primary care

POC tests are useful when a rapid result changes today’s action. They are harmful when a poor indication produces a false path.

POC tests do not replace HPI. A rapid strep on a child with cough and runny nose creates a false-positive treatment path. A lead capillary on a child with pica does belong on a sick visit.

Laboratory studies: order to answer a question

CBC. Use it when the differential includes anemia, leukemia, infection with systemic features, unexplained bruising, or a limp that might be marrow disease. It is not a well-child ritual. Pediatric interpretation is age-specific: newborns have high hemoglobin; a physiologic nadir occurs in the first months of life; lymphocyte predominance is normal in many toddlers. PNCB will not hand you an adult reference table.

Ferritin. Iron deficiency can exist with a still-normal hemoglobin. Ferritin reflects stores; inflammation can raise it, so interpret with the clinical picture (and CRP when inflammation is likely). Fatigue, pica, excessive milk intake, and food insecurity are reasons to think beyond a POC hemoglobin.

TSH and free T4. Order when growth velocity has fallen, goiter is present, constipation plus bradycardia plus poor growth cluster, or a condition with surveillance guidance (for example, Down syndrome health supervision) is on the problem list. Do not add thyroid tests to every overweight teen without a hypothesis.

Lipids — Bright Futures universal versus selective. Teach the periodicity principle, not a made-up year. AAP/Bright Futures and the NHLBI pediatric framework use universal lipid screening once in late childhood (9–11 years) and once in late adolescence (17–21 years). Selective screening is used at other ages (including from age 2) when family history of early atherosclerotic disease or familial hypercholesterolemia, or the child has diabetes, hypertension, obesity, or other risk conditions. Non-HDL or a fasting lipid profile may be used per the guideline you are following; the exam tests that universal late-childhood and late-adolescent screens exist, not that you invent a third birthday lipid stick for every toddler.

Urinalysis and culture. Symptomatic children and selected febrile infants: collect well (catheter or SPA in young infants; clean catch when realistic). Asymptomatic bacteriuria is not a reason to shotgun antibiotics.

STI NAAT. Nucleic-acid amplification for gonorrhea and chlamydia (urine or swab) is the primary-care workhorse in sexually active adolescents. Offer it in confidential care consistent with state minor-consent law. A wet mount or POC testing may add trichomonas where available; HIV and syphilis follow current adolescent screening principles. Do not condition an indicated NAAT on parental presence.

Newborn-screen follow-up. An out-of-range newborn screen is a same-day workflow, not a “repeat at the next well visit.” Contact the family, follow the state program’s confirmatory diagnostic instructions (which may be a specific blood test, not another filter paper), and involve the designated specialist. Prematurity and timing of the specimen cause false positives; false positive is not “ignore.” Some metabolic results require feeding and emergency instructions while you wait for confirmation.

Imaging: when not to image, and radiation stewardship

Most acute bacterial sinusitis is a clinical diagnosis. Persistent, worsening, or severe-onset symptoms drive treatment decisions. Do not CT the sinuses for routine rhinosinusitis. Image when you suspect complication (orbital cellulitis, intracranial extension) or when a surgeon needs anatomy — that is not a Friday afternoon “confirm sinusitis” film.

First simple febrile seizure. In a 6-month to 5-year-old who had a brief, generalized seizure with fever, returned to baseline, and has a nonfocal exam, AAP guidance does not support routine laboratory panels, routine EEG, or routine neuroimaging. Do not CT “just in case.” LP is individualized (meningeal signs, very young or incompletely immunized infants, pretreated with antibiotics). Imaging belongs to focal findings, prolonged altered consciousness, or another red flag — not to every first simple febrile seizure.

Radiation stewardship is ALARA (as low as reasonably achievable). Prefer ultrasound or MRI when they answer the question. Do not stack CT on a well-appearing child because the family “wants to be sure.”

Pediatric “normals” without a printed reference range

Because PNCB generally does not print lab reference ranges, answers that require a memorized adult cutoff the stem never gave are usually wrong. Conceptual anchors:

• Hemoglobin is high at birth, falls to a physiologic nadir in early infancy, then rises; childhood cutoffs differ from adult men.
• Alkaline phosphatase is higher in growing children than in adults because of bone turnover; a “high ALP” in a healthy 8-year-old is not automatically cholestasis.
• WBC and differential shift with age; lymphocyte predominance in a toddler is often normal, not “viral proof” by itself.
• Creatinine is lower in small children; “normal adult creatinine” can still be kidney injury in a toddler.

Interpret trend and clinical context. If the stem gives a value and an age, use pediatric reasoning; if it does not, do not invent a number.

Consent and assent for invasive tests

Minors need parental permission (consent) for most testing, plus the child’s assent when developmentally able: explain the poke, the urine catheter, or the imaging in age-appropriate language, allow questions, and do not treat a screaming toddler as if explanation were optional. Adolescents may independently consent to indicated STI testing and selected reproductive care under state law — know the principle that confidential indicated NAAT is not withheld because a parent left the room. Emergency testing when delay threatens life or limb follows emergency-exception logic; you still explain as soon as you can. Document who consented, what the child was told, and any refusal.

On the exam: order the test that answers the leading hypothesis, skip the CT that does not, confirm an abnormal capillary lead, and never delay torsion surgery for a pretty ultrasound.