Diagnostic & Monitoring Terms

Standard test (high-dose): 250μg cosyntropin IV or IM → serum cortisol at 0, 30, 60 minutes. Normal response: cortisol ≥18-20 μg/dL (500-550 nmol/L) at 30 or 60 minutes. Subnormal response indicates adrenal insufficiency. Low-dose test: 1μg cosyntropin IV → may be more sensitive for detecting mild/early secondary adrenal insufficiency (subtle ACTH deficiency from recent pituitary surgery or gradual glucocorticoid withdrawal). Limitations: the standard 250μg dose is supraphysiological (pharmacological rather than physiological stimulation), which may overcome mild adrenal insufficiency — hence the low-dose alternative. Timing considerations: for secondary adrenal insufficiency (pituitary/hypothalamic cause), adrenal atrophy from prolonged ACTH deficiency takes weeks-months to develop, so the ACTH stimulation test may be falsely normal in acute pituitary damage (within 2-4 weeks). The insulin tolerance test remains the gold standard for secondary adrenal insufficiency but carries hypoglycaemia risks.

Serum amylase: produced by pancreas (P-isoamylase, ~40% of total) and salivary glands (S-isoamylase, ~60%). Elevated amylase: acute pancreatitis (typically >3× ULN, peaks at 12-72 hours, returns to normal in 3-5 days), salivary disease (mumps, parotitis), bowel obstruction, renal failure (reduced clearance), macroamylasaemia (amylase-immunoglobulin complex — falsely elevated, benign). Amylase is less specific for pancreatitis than lipase — amylase can be elevated from multiple non-pancreatic sources. For GLP-1 RA safety monitoring: routine amylase/lipase screening is not required, but these tests should be obtained promptly if a patient develops persistent severe abdominal pain (potential pancreatitis). In GLP-1 RA clinical trials, mild amylase/lipase elevations (without clinical pancreatitis) are observed more frequently in treated vs placebo groups — clinical significance is debated.

FDA/NIH BEST (Biomarkers, EndpointS, and other Tools) resource classifies biomarkers: diagnostic (identifying disease — HbA1c ≥6.5% for diabetes), prognostic (predicting disease outcome regardless of treatment — NT-proBNP in heart failure), predictive (predicting response to specific treatment — Ga-68 DOTATATE uptake predicting Lu-177 PRRT response), pharmacodynamic (measuring drug effect — HbA1c change reflecting GLP-1 RA glycaemic effect), safety (monitoring adverse effects — calcitonin for C-cell tumour surveillance during GLP-1 RA therapy), and monitoring (serial measurement detecting disease change — IGF-1 during GH or somatostatin analogue therapy). Biomarker qualification by regulatory agencies (FDA Biomarker Qualification Program, EMA qualification advice) validates specific contexts of use. For peptide drug development, validated biomarkers can serve as surrogate endpoints, potentially accelerating development timelines through earlier efficacy assessment.

BMD is measured in g/cm² and compared to reference populations: T-score (comparison to young adult peak bone mass — used for postmenopausal women and men ≥50) and Z-score (comparison to age/sex-matched population — used for premenopausal women, men <50, and children). DEXA measurement sites: lumbar spine L1-L4 (cancellous bone predominant — responsive to treatment changes), femoral neck (predictive of hip fracture), total hip (integrated cortical/cancellous), and distal radius (when spine/hip sites are invalid). For PTH analogue therapy (teriparatide, abaloparatide): spine BMD increases approximately 8-13% over 18-24 months, with smaller hip BMD increases. Monitoring interval: typically 1-2 years during treatment. Minimum significant change (MSC) at each DEXA site determines whether a real BMD change has occurred vs measurement variability — typically approximately 3-5% at the spine and 4-6% at the hip.

CRP is an acute-phase protein synthesised by hepatocytes in response to IL-6 stimulation. It rises rapidly (within 6-8 hours) and dramatically (up to 1000-fold) during acute infection/inflammation, then falls quickly (half-life ~19 hours) when inflammation resolves. High-sensitivity CRP (hs-CRP): measures low-level CRP (0.1-10 mg/L range) as a cardiovascular risk biomarker — hs-CRP <1 mg/L (low risk), 1-3 mg/L (moderate), >3 mg/L (high cardiovascular risk). hs-CRP reflects the chronic low-grade inflammation underlying atherosclerosis. GLP-1 RAs may reduce hs-CRP through: weight loss (reducing inflammatory adipokine production), improved insulin sensitivity, and potential direct anti-inflammatory effects. Some peptide drug trials include hs-CRP as a secondary endpoint reflecting the anti-inflammatory component of metabolic improvement.

Serum calcitonin measurement: by immunometric assay (chemiluminescent or electrochemiluminescent). Reference ranges: typically <10 pg/mL for women and <17 pg/mL for men (assay-specific). Elevated calcitonin: medullary thyroid carcinoma (MTC — calcitonin >100 pg/mL is highly suspicious; >500 pg/mL is virtually diagnostic), C-cell hyperplasia (MTC precursor — moderately elevated), renal failure (reduced clearance), proton pump inhibitors (mild elevation from gastrin-mediated C-cell stimulation), and various non-thyroid cancers (rare). Regarding GLP-1 RA therapy: routine calcitonin screening before or during GLP-1 RA therapy is NOT recommended by clinical guidelines (ADA, Endocrine Society) because: MTC is very rare (~0.01%), calcitonin screening has low positive predictive value (many false positives causing unnecessary workups), and the C-cell tumour signal is from rodent studies with uncertain human relevance. However, if calcitonin is incidentally elevated, thyroid ultrasound and endocrine referral are warranted.

CBC components: red blood cells (RBC count, haemoglobin, haematocrit, MCV, MCH, MCHC, RDW), white blood cells (total WBC, differential: neutrophils, lymphocytes, monocytes, eosinophils, basophils), and platelets (count, MPV). For peptide drug monitoring: romiplostim (platelet count monitoring — target ≥50,000/μL; excessive response risks thrombosis), elamipretide (neutrophil count — monitoring for Barth syndrome-associated neutropenia improvement), carfilzomib/bortezomib (monitoring for chemotherapy-induced cytopaenia — thrombocytopenia, neutropenia, anaemia), and GnRH agonists (monitoring androgen deprivation effects — anaemia from testosterone suppression). CBC with peripheral blood smear is essential for: detecting thrombotic microangiopathy (potential complication of some biologics), monitoring reticulin fibrosis risk with romiplostim (schistocytes on smear may indicate developing fibrosis), and overall safety surveillance during peptide drug therapy.

CT acquires cross-sectional X-ray images reconstructed by computer into detailed 2D/3D images. Quantitative CT (QCT) can measure volumetric BMD (mg/cm³) at the spine — distinguishing cortical from trabecular bone (more sensitive to metabolic bone disease than DEXA areal BMD). CT is used diagnostically in peptide drug contexts for: pituitary imaging (detecting adenomas in acromegaly/Cushing's — though MRI is preferred for pituitary), abdominal visceral fat quantification (research standard for metabolic studies — measuring VAT area at L4-L5 in cm²), NET staging (detecting liver metastases, lymphadenopathy), and bone fracture assessment. CT-guided procedures: stereotactic pituitary biopsy, and CT-guided brachytherapy for certain cancers. Limitation: ionising radiation (effective dose 2-20 mSv depending on study) restricts use in serial monitoring, particularly in children and young adults.

DEXA technology: two X-ray beams at different energy levels (typically 40 keV and 70-100 keV) pass through the body; differential attenuation by bone and soft tissue allows calculation of bone mineral content (BMC, in grams) and bone area (in cm²), yielding areal BMD (BMC/area, in g/cm²). DEXA also measures body composition: fat mass, lean mass, and bone mineral content for whole body and regional (android, gynoid, arms, legs, trunk) analysis. Radiation dose: very low (~1-10 μSv per scan, comparable to 1-2 days of background radiation). Precision: coefficient of variation approximately 1-1.5% at spine, 1.5-2.5% at hip. Sources of error: degenerative changes (osteophytes artificially elevate spine BMD), aortic calcification, fracture compression, obesity (affecting beam hardening). For clinical trials of bone-active peptide drugs, DEXA is the standard endpoint for demonstrating BMD changes, with fracture incidence as the definitive clinical endpoint.

Ga-68 DOTATATE (NETSPOT, Detectnet): Ga-68 chelated to DOTA-Tyr3-octreotate, binding SSTR2 with high affinity. PET/CT protocol: IV injection of approximately 2 MBq/kg → 60-minute uptake period → whole-body PET/CT acquisition. Image interpretation: physiological uptake in pituitary, adrenals, spleen, liver, kidneys, and head of pancreas (SSTR-expressing normal tissues); pathological uptake in tumour lesions quantified by SUVmax. Clinical applications: initial NET staging, restaging after treatment, detection of unknown primary, and selection for Lu-177 PRRT (patients with adequate tumour uptake — typically SUVmax greater than hepatic background — are candidates for PRRT). The Krenning score (visual comparison of tumour uptake to reference organs) was developed for OctreoScan but adapted for DOTATATE PET. Ga-68 DOTATATE PET has changed NET management by detecting lesions missed by conventional imaging in approximately 30-40% of patients, altering treatment decisions in approximately 30% of cases.

Stimulation tests are required because GH is secreted pulsatily with low baseline levels — a random GH level is uninformative. Standard stimuli: ITT (gold standard — tests hypothalamic-pituitary-adrenal AND somatotroph function), GHRH-arginine (tests pituitary GH reserve directly; arginine suppresses somatostatin, GHRH stimulates; BMI-adjusted cutoffs: BMI<25: GH ≥11, BMI 25-30: GH ≥8, BMI>30: GH ≥4 μg/L), glucagon stimulation test (1mg IM glucagon → GH and cortisol measured over 3-4 hours; cutoff GH ≥3 μg/L), and macimorelin test (oral GHS-R agonist — 0.5mg/kg oral solution → blood GH at 30, 45, 60, 90 minutes; cutoff ≥2.8 μg/L; FDA-approved specifically for adult GHD diagnosis, offering convenience of oral administration). Paediatric GHD requires two failed stimulation tests (to reduce false positive diagnoses). GH stimulation testing establishes the diagnosis that justifies somatropin, somapacitan, or somatrogon therapy.

75g OGTT protocol: overnight fast → fasting blood glucose → 75g anhydrous glucose dissolved in 300mL water consumed over 5 minutes → blood glucose at 1 hour (for GDM screening) and 2 hours. Diagnostic criteria (WHO/ADA): normal (2h glucose <140 mg/dL), impaired glucose tolerance (2h 140-199 mg/dL), diabetes mellitus (2h ≥200 mg/dL). For GDM (IADPSG criteria): fasting ≥92, 1h ≥180, 2h ≥153 mg/dL (any one value diagnostic). In acromegaly: 75g OGTT with serial GH measurements — failure to suppress GH below 1 μg/L (or 0.4 μg/L with ultrasensitive assays) confirms autonomous GH secretion. After treatment (surgery or somatostatin analogue), repeat OGTT assesses biochemical remission. The OGTT also has research applications in metabolic peptide drug trials: characterising glucose disposition (insulin sensitivity and beta cell function from OGTT-derived indices: Matsuda index, insulinogenic index, oral disposition index).

ITT protocol: 0.1-0.15 U/kg regular insulin IV → adequate hypoglycaemia must be achieved (glucose <40 mg/dL or <2.2 mmol/L with symptoms) → GH and cortisol measured at 0, 30, 60, 90, 120 minutes. Normal responses: GH ≥5 μg/L (some use ≥3 μg/L cutoff), cortisol ≥18-20 μg/dL. ITT advantages: tests both GH and cortisol axes simultaneously, gold standard for adult GHD diagnosis. Contraindications: epilepsy, ischaemic heart disease, adrenal insufficiency (known — could be dangerous), and elderly patients. Safety requirements: physician supervision throughout, IV glucose immediately available, minimum glucose of 40 mg/dL confirmed with point-of-care testing. The ITT has been partially replaced by safer alternatives: GHRH-arginine test for GH axis (but depends on GHRH availability), glucagon stimulation test (for both GH and cortisol — cosyntropin tests cortisol axis separately), and macimorelin test (oral GHS-R agonist specifically for GHD diagnosis, FDA-approved).

Serum lipase: produced almost exclusively by the pancreas (more specific than amylase for pancreatic disease). In acute pancreatitis: lipase rises within 4-8 hours of onset, peaks at 24 hours, and remains elevated for 8-14 days (longer than amylase). Lipase >3× ULN has approximately 95% sensitivity and specificity for acute pancreatitis. Elevated without pancreatitis: renal failure, diabetic ketoacidosis, bowel obstruction, and some medications. For GLP-1 RA safety: the pancreatitis safety signal is monitored through both clinical reporting (symptoms) and biochemical surveillance (lipase/amylase in some trials). Isolated lipase elevation without symptoms (incidental finding during monitoring) is common with GLP-1 RAs and does not necessarily indicate pancreatitis — clinical context is essential. Guidelines recommend discontinuing GLP-1 RA if acute pancreatitis is confirmed (not merely for asymptomatic enzyme elevation).

Standard lipid panel: total cholesterol, LDL-C (calculated by Friedewald equation or direct measurement), HDL-C, and triglycerides. Advanced panels may include: non-HDL-C (total minus HDL — includes all atherogenic particles), apoB (one molecule per atherogenic particle — most accurate measure of particle number), Lp(a) (genetically determined atherogenic lipoprotein), and LDL particle size/number. For metabolic peptide drug trials: lipid parameters are standard secondary endpoints. GLP-1 RA effects: triglyceride reduction approximately 10-20%, LDL-C reduction approximately 0-5%, HDL-C increase approximately 1-3%, total cholesterol reduction approximately 3-8%. Tirzepatide showed somewhat larger lipid improvements. Somatostatin analogues can worsen lipid profiles (by reducing insulin secretion and affecting hepatic lipid metabolism). Lipid monitoring is standard at baseline and periodically during metabolic peptide drug therapy, particularly in patients with pre-existing dyslipidaemia.

Standard LFT panel: ALT (alanine aminotransferase — most specific for hepatocellular damage), AST (aspartate aminotransferase — hepatocellular damage, also elevated in muscle/cardiac injury), ALP (alkaline phosphatase — cholestatic disease, bone disease), GGT (gamma-glutamyl transferase — cholestatic disease, alcohol-induced liver injury), total and direct bilirubin (jaundice, haemolysis), and albumin (synthetic function — reduced in chronic liver disease). For peptide drugs: pasireotide can cause hepatic enzyme elevation; somatostatin analogues can cause cholelithiasis (gallstones — reduced gallbladder motility) requiring periodic liver/biliary monitoring; cyclosporine requires LFT monitoring for hepatotoxicity; and carfilzomib/bortezomib may cause transient transaminase elevation. Baseline LFTs and periodic monitoring (every 3-6 months) are standard during many peptide drug therapies.

MRI uses strong magnetic fields and radiofrequency pulses to align and detect hydrogen nuclei in water and fat. Advantages: no ionising radiation, excellent soft tissue contrast, multiplanar imaging. MRI is the primary imaging for: pituitary adenomas (T1-weighted with gadolinium contrast — microadenomas appear as hypointense foci; sensitivity approximately 90% for macroadenomas), brain disorders (MS — FLAIR sequences detect white matter lesions; neurodegenerative diseases — hippocampal atrophy), soft tissue injuries (tendon/ligament assessment relevant to tissue repair peptide research), and body composition (MRI-based volumetric fat quantification — most accurate method for visceral fat measurement, used in metabolic drug trials). Limitations: cost, availability, claustrophobia, contraindications (pacemakers, ferromagnetic implants), and long acquisition times. MRI enterography is used for small bowel Crohn's disease assessment (relevant to SBS patients treated with teduglutide).

NT-proBNP is produced 1:1 with BNP from proBNP cleavage but has a longer half-life (~120 min vs ~20 min for BNP) and higher circulating concentrations, making it easier to measure reliably. Age-stratified rule-out cutoffs for acute heart failure (ESC guidelines): <50 years: <450 pg/mL; 50-75 years: <900 pg/mL; >75 years: <1800 pg/mL. Single rule-out cutoff: <300 pg/mL (high negative predictive value ~98%). Elevated NT-proBNP also occurs in: renal impairment (reduced clearance), pulmonary embolism, sepsis, critical illness, and atrial fibrillation. For peptide drug trials: NT-proBNP is increasingly used as a safety biomarker (detecting subclinical cardiac effects) and as an inclusion/stratification criterion for cardiovascular outcome trials. The SELECT trial (semaglutide 2.4mg for cardiovascular risk reduction) used NT-proBNP as a biomarker of cardiac risk.

PET detects gamma rays from positron-emitting radiotracers. The radiotracer is injected IV → distributes to tissues → positron emission → annihilation with electron → two 511keV gamma photons at 180° → detected by ring detector. Common tracers: FDG (18F-fluorodeoxyglucose — glucose metabolism, used in oncology for tumour staging), Ga-68 DOTATATE (somatostatin receptor imaging — the companion diagnostic for Lu-177 dotatate PRRT), Ga-68 PSMA (prostate cancer imaging — companion for Lu-177 vipivotide tetraxetan), and various research tracers. Ga-68 DOTATATE PET/CT: sensitivity >90% and specificity >95% for SSTR-positive NETs — superior to conventional CT/MRI and older scintigraphy (OctreoScan). The theranostic paradigm: same targeting peptide (DOTATATE) used with diagnostic isotope (Ga-68, positron emitter for PET) and therapeutic isotope (Lu-177, beta emitter for PRRT), enabling selection of patients most likely to benefit from targeted radionuclide therapy.

Key parameters: serum creatinine (muscle-derived waste product filtered by kidneys — elevated when GFR falls), eGFR (estimated from creatinine using CKD-EPI equation incorporating age, sex, and race — normal >90 mL/min/1.73m²; CKD staged by eGFR: G1 ≥90, G2 60-89, G3a 45-59, G3b 30-44, G4 15-29, G5 <15), BUN/urea (less specific than creatinine), cystatin C (alternative filtration marker, less affected by muscle mass), and urine albumin:creatinine ratio (UACR — detecting albuminuria, an early marker of kidney damage). For peptide drugs: dose adjustment required for renally-cleared peptides in CKD (colistin, polymyxin B, vancomycin — requiring TDM); difelikefalin is specifically dosed for haemodialysis patients; GLP-1 RAs generally do not require dose adjustment in mild-moderate CKD but caution with dehydration risk from GI side effects; semaglutide FLOW trial investigated kidney outcomes in T2D patients with CKD.

IGF-1 reference ranges are age- and sex-specific (expressed as μg/L or nmol/L, with SDS/Z-score for comparison to reference population). Peak levels occur during puberty (approximately 300-500 μg/L), declining to approximately 100-200 μg/L in middle age and 50-100 μg/L in elderly. Clinical applications: GHD diagnosis (low IGF-1 supports clinical suspicion but is not sufficient alone — approximately 30-50% of adults with GHD have IGF-1 in the normal range), acromegaly diagnosis and monitoring (elevated IGF-1 confirms GH excess; target normalisation during treatment with somatostatin analogues), GH replacement dose titration (titrate somatropin/somapacitan/somatrogon to maintain IGF-1 in upper half of age-appropriate normal range), and safety monitoring (IGF-1 above normal range during GH therapy increases theoretical cancer risk — though this remains debated). Pre-analytical variables: fasting state, nutritional status, liver function, and oestrogen status affect IGF-1 levels.

OctreoScan (111In-pentetreotide/111In-DTPA-octreotide): radiolabelled octreotide analogue imaged by gamma camera/SPECT. Sensitivity: approximately 70-90% for well-differentiated NETs (lower for G3 NETs and insulinomas which may have lower SSTR expression). OctreoScan was the standard imaging for NETs before Ga-68 DOTATATE PET/CT became available. Ga-68 DOTATATE PET/CT advantages: higher spatial resolution (PET vs SPECT), higher sensitivity (>90% vs ~80%), lower radiation dose (Ga-68 shorter half-life), same-day imaging (vs 24-48 hour delay with 111In), and quantitative uptake measurement (SUV values predicting PRRT response). OctreoScan remains available where PET/CT is unavailable but is being replaced by Ga-68 DOTATATE PET/CT as the standard of care for NET imaging worldwide.

T-score = (patient's BMD − young adult mean BMD) / young adult SD. WHO classification: normal (T ≥ −1.0), osteopenia (−2.5 < T < −1.0), osteoporosis (T ≤ −2.5), and established/severe osteoporosis (T ≤ −2.5 with fragility fracture). For every 1.0 SD decrease in BMD, fracture risk approximately doubles. However, T-score alone has limited predictive power for individual fracture risk — most fractures occur in patients with osteopenia rather than osteoporosis (because many more people have osteopenia). FRAX integration: combining T-score with clinical risk factors provides better fracture prediction than T-score alone. Treatment thresholds vary by country: US (NOF/AACE) — treat if T ≤ −2.5 OR fragility fracture OR FRAX ≥3% hip/≥20% major osteoporotic; UK (NOGG/NICE) — FRAX-based, age-dependent thresholds. PTH analogue therapy is generally reserved for patients with T-scores well below −2.5, multiple fractures, or failed anti-resorptive therapy.

TDM is routinely performed for: vancomycin (target trough 15-20 μg/mL for serious infections, 10-15 μg/mL for less serious; AUC/MIC-based dosing increasingly preferred — target AUC24 400-600 mg·h/L for MRSA), cyclosporine (trough levels: target varies by indication and time post-transplant — typically 150-300 ng/mL early post-transplant, 100-200 ng/mL maintenance), and aminoglycoside antibiotics (if combined with peptide antibiotics). TDM is NOT routinely required for most peptide drugs: GLP-1 RAs (predictable PK, dose-titrated by clinical response), somatostatin analogues (clinical and biochemical response monitoring — IGF-1, not drug levels), and GnRH compounds (monitored by LH, FSH, and sex steroid levels rather than drug concentrations). TDM principles: sampling at steady state (after 4-5 half-lives), correct timing (trough = pre-dose; peak timing depends on route), validated assay methodology, and clinical correlation (drug levels interpreted alongside clinical response and toxicity signs).