Advances in the use of dried blood spots on filter paper to monitor kidney disease

Dry blood spots to monitor kidney disease

Article information

Child Kidney Dis. 2024;28(1):16-26

Publication date (electronic) : 2024 February 28

doi : https://doi.org/10.3339/ckd.23.023

Carla Nicola ¹

, Vandréa de Souza ¹

¹Graduate Health Sciences Program, Universidade de Caxias do Sul, Caxias do Sul, Brazil

Correspondence to: Carla Nicola Graduate Health Sciences Program, Universidade de Caxias do Sul, Francisco Getúlio Vargas, 1130 Caxias do Sul 95070-560, Brazil E-mail: carla1982n@hotmail.com

Received 2023 November 24; Revised 2024 January 23; Accepted 2024 February 9.

Abstract

Patients with kidney disease require frequent blood tests to monitor their kidney function, which is particularly difficult for young children and the elderly. For these people, the standard method is to evaluate serum creatinine or cystatin C or drug levels through venous sampling, but more recently, evaluation using dried blood spots has been used. This narrative review reports information from the literature on the use of dried blood spots to quantify the main markers used to detect kidney diseases. The ScienceDirect and PubMed databases were searched using the keywords: "dried blood on filter paper," "markers of renal function," "renal function," "creatinine," "cystatin C," "urea," "iohexol," and "iotalamate." Studies using animal samples were excluded, and only relevant articles in English or Spanish were considered. Creatinine was the most assessed biomarker in studies using dried blood spots to monitor kidney function, showing good performance in samples whose hematocrit levels were within normal reference values. According to the included studies, dried blood spots are a practical monitoring alternative for kidney disease. Validation parameters, such as sample and card type, volume, storage, internal patterns, and the effects of hematocrit are crucial to improving the reliability of these results.

Keywords: Creatinine; Cystatin C; Dried blood spot testing; Iohexol; Urea

Introduction

Kidney disease is a public health problem, with more than 750 million people diagnosed worldwide. In 2019, 1.3 million people lost their lives due to kidney failure, and nearly 1.7 million die from acute kidney injury every year [1-3]. Chronic kidney disease (CKD) is a challenge because it manifests with unspecific or no clinical symptoms; symptoms are detected only at more advanced stages [4,5]. The most frequent complications of this disease are cardiovascular disorders, mineral and bone imbalance, and progression of CKD [6].

Kidney diseases can be recognized by identifying an imbalance in markers such as amino acids, lipids, and nucleotides. These compounds can suggest that there is a problem, expediting proper treatment and thus reducing complications [7-9]. The main indicators of kidney injury are albuminuria (albumin to creatinine ratio ≥30 mg/g), urinary sediment abnormalities characteristic of tubular disease, electrolytic disorders, and reduced renal function (glomerular filtration rate [GFR] <60 mL/min/1.73 m²) [10-12].

Indicators of kidney injury are mainly detected through urine and venous blood samples. These samples must be refrigerated due to the instability of the compounds, which can undergo enzymatic degradation [13,14]. However, dried blood spots (DBS) have gained relevance and may especially benefit populations at risk of CKD [13,15]. DBS is advantageous for infants and elderly patients, especially because it requires less blood volume than conventional tests [16,17]. Despite the observed practical advantages, these assays involve methodological concerns that should be discussed, such as homogeneity of the sample point, hematocrit, and sample recovery [18,19].

Hence, this narrative review covers potential applications of DBS including GFR estimation, drug level monitoring, and its advantages and limitations, as well as precautions when applying it in clinical practice.

Markers of kidney function

The GFR, which describes the volume of plasma filtered from the glomerular capillaries into Bowman's capsules per unit of time [20], is considered a sensitive and specific indicator of abnormal kidney function [21]. The gold standard for GFR measurement is determining the clearance of compounds filtered exclusively by the glomeruli. Exogenous markers, such as iohexol, inulin, and iothalamate, meet this criterion, but they are used only in specific situations (e.g., drug adjustment or kidney protocols) due to their cost and complexity [10].

In most circumstances, GFR is estimated using compounds eliminated by the kidneys (creatinine and cystatin C) based on mathematical equations to correct for biological variations [22,23]. Creatinine levels vary according to age, sex, metabolism, muscle mass, and nutritional status. Cystatin C seems to be less dependent on biological factors, but its levels may increase with glucocorticoid use and show poor agreement during pregnancy due to placental production [8,24,25].

Principles and applications of DBS methods

The first officially established tests to use dried whole blood samples on filter paper in the pre-analytical phase of laboratory testing were performed in 1963, with the discovery of an effective low-cost neonatal screening test to identify phenylketonuria [26]. The successful screening of this and other inborn errors of metabolism using DBS has led to its adaptation for a myriad of analytical parameters, such as drug monitoring, protein studies, and infectious disease management [14,27,28]. Table 1 summarizes the main applications of DBS in kidney diseases.

Table 1.

Potential applications of dried blood spots in kidney disease

The filter paper method has advantages over conventional venipuncture, since blood collection is easy to perform, less invasive, and relatively painless [29,30]. The paper filter method minimizes the volume of blood taken from patients and may be performed without specialized structures [29]. Furthermore, it is better suited for clinical research and patients who must undergo numerous blood tests or who have damaged veins, as well as for infants and older people [29,31,32].

Determining biochemical parameters from blood samples requires a well-established quality control system [33]. Factors such as sample collection procedure, sample volume, spot quality, filter paper type, drying and storage methods, hematocrit, and the incorporation of internal standards are important parameters for good DBS performance and vary depending on the analyte [34-37].

Relevant factors in DBS methods

Sample collection

In the classic filter paper system, a few drops of whole blood (5–50 μL) are collected on a card by finger prick with a lancet [29]. At this stage, certain precautions are essential, such as thorough disinfection, discarding the first drop of blood, which may contain tissue fluid, completely filling in the card’s outlined circle, and drying the sample at room temperature [14,38]. In viability testing of home-collected DBS samples for creatinine analysis, blood adherence to the cards was high, but only 80% of the spots showed accurate saturation and were suitable for analysis [39].

Capillary blood collected by finger prick is a mixture of arterial blood, venous blood, and interstitial fluids. Biomarker concentrations in capillary blood collected in DBS should be different from those found in venous blood [35]. Lower concentrations of cystatin C were found in blood collected by finger prick than in venous blood [40]. GFR measured by iohexol clearance has proven reliable in venous samples and capillary blood spots, although the capillary method overestimated venous GFR by 7.2% [41]. Conversely, both venous sampling and finger stick sampling at 2-time points after iohexol infusion resulted in an acceptably accurate GFR measurement [42]. Variability in creatinine levels between capillary and venous blood samples was compared using the gold standard method, isotope dilution mass spectrometry, which reinforced the importance of using correction factors derived from validation studies to align the values obtained through each method [43].

Filter paper

The filter paper type may affect the homogeneity and behavior of blood spreading, as well as compound stability and recovery [35,44]. The main types of filter paper are made of cellulose (Whatman, GE Healthcare and Ahlstrom, Perkin-Elmer) or glass microfiber (Agilent Bond Elut DMS and Sartorius) [29,38].

Cellulose-based cards may contain additives, such as enzyme inhibitors or denaturing agents [35,38]. Whatman FTA DMPK-A cards are impregnated with radical inhibitors [sodium dodecyl sulfate, tris(hydroxymethyl) aminomethane] and can promote cell lysis and denature proteins on contact. Similarly, Whatman FTA DMPK-B cards are impregnated with chaotropic agents (guanidinium thiocyanate). Cotton-based cards, such as Whatman FTA DMPK-C, are not impregnated with stabilizing materials and are suitable for protein analysis, as are Whatman 903 and Ahlstrom 226 [33].

Due to the range of available filter cards, the European Bioanalysis Forum recommends fully validating DBS sampling methods for specific paper types [45,46]. Recommended validation parameters include drying conditions, storage stability, the effects of sample recovery, linearity, accuracy, and precision [46].

Hematocrit

Hematocrit variability is the main factor affecting the quality of DBS results [47]. Hematocrit reflects the relative volume of red blood cells and affects blood viscosity. High hematocrit results in low absorption into the card [31]. Human reference values vary according to biological parameters such as age, sex, nutritional status, race, pathological conditions, and pregnancy, in addition to extrinsic factors, such as altitude and smoking [47]. Mathematical equations to correct these variations have been determined based on the patient's baseline value or reference values for men and women [14]. Using computer systems to apply specific correction factors based on demographic data may help correct the impact of hematocrit on DBS measurements and achieve accurate analytical results. However, for precision, many sources of random errors (pipettes, volumetric flasks, detector, extraction procedure) must be accounted for [47].

The effect of hematocrit depends on the analyte of interest, and different results may be obtained according to its physical and chemical properties [48,49]. This effect can be measured either directly or indirectly through endogenous compounds such as sphingomyelin and potassium [47,50]. Incorporating internal standards, in association with accurate volume sampling, whole-spot extraction, and automated direct elution techniques has been shown to minimize the effect of hematocrit and thus improve reliability [51,52].

In studies involving individuals with abnormal hematocrit levels, DBS sampling proved unsuitable for iothalamate analysis [53]. Low hematocrit also significantly influenced creatinine analysis (deviation of 15%), and correction with endogenous compounds (potassium) was suggested [50]. Conversely, some studies reported that hematocrit’s effects on precision were within acceptable limits [32,54,55].

Applicability of the DBS technique in nephrology

Measurement of endogenous markers

Using DBS to quantify endogenous markers of kidney function has mainly occurred in the last decade (Table 2) [13,34,40,43,56-63]. A strong correlation was found between conventionally obtained venous blood samples and those collected through DBS [43,57,58]. Using the reference method, creatinine quantification in DBS samples showed good accuracy [58]. Nevertheless, only Dalton et al. [43] compared creatinine levels in whole capillary DBS samples (n=66) using isotope dilution mass spectrometry.

Table 2.

Studies involving measurement of endogenous biomarkers of kidney function through DBS samples

One observed advantage of DBS is the stability of compounds. Creatinine showed 7-day stability at 32 °C in blood collected on Whatman FTA DMPK-C cards [32]. Quraishi et al. [56] also reported that creatine is stable for up to 90 days between 4 °C and 37 °C in serum samples stored on filter discs. Similarly, DBS urea concentrations were stable for up to 120 days at 4 °C and for 90 days at 37 °C [63]. However, cystatin C values decreased when shipping times exceeded 8 days (n=3,149) [34].

Measurement of exogenous markers

To determine the GFR through the clearance of exogenous compounds, blood must be collected several times over specific periods [64]. The filter paper method could simplify this process and be more tolerable in special populations, such as children [42]. Table 3 shows the main studies that have assessed methods of measuring exogenous markers of kidney function through DBS [41,42,53,65-69]. As found in a previous study, there was strong agreement between DBS and venous GFR, with acceptable bias, precision, and accuracy, especially in patients with GFR <60 mL/min/1.73 m² [41]. Linear regression analyses also found good agreement between 82 serum and DBS samples regarding iohexol concentration [65].

Table 3.

Studies involving measurement of exogenous markers of kidney function through DBS samples

Serum medication levels

Simultaneous assessment of kidney function indicators and medications in filter paper collection systems is a promising method for controlling the clearance or toxicity of drugs or their metabolites [70]. Forms of nephrotoxicity include tubular epithelial cell injury (antimicrobials, chemotherapeutic drugs, and venous contrast agents), interstitial nephritis (antibiotics, anti-inflammatory drugs, proton pump inhibitors, and immune-checkpoint inhibitors), and the formation of intratubular crystals (acyclovir, indinavir, antimicrobials, methotrexate, and sulfadiazine) [71].

Risk factors, such as advanced age, cardiovascular disease, diabetes, and liver disease, contribute to the development of kidney dysfunction after nephrotoxic drug use [72]. Combined therapies with diuretics, non-steroidal anti-inflammatory drugs, and renin angiotensin system inhibitors may potentiate nephrotoxicity in this group of patients [73]. Since drugs can also accumulate when kidney function is reduced (digoxin, metformin, and lithium), periodical kidney function assessment is needed in these patients [74,75].

Good correlations have been observed between serum and DBS samples for creatinine and immunosuppressant quantification by liquid chromatography-tandem mass spectrometry (Table 4) [32,39,54,74,76-80]. Simultaneous analysis of creatinine and diabetes medications (metformin and sitagliptin) has also shown good accuracy and precision in DBS samples [74,76]. Cystatin C-based measures of renal function improved ceftriaxone clearance prediction in 26 elderly patients [81]. Conversely, vancomycin clearance levels could not be accurately predicted through DBS [54].

Table 4.

Studies that simultaneously measured creatinine and medication clearance through DBS samples

Kidney transplant patients also require constant kidney function assessment, in addition to effective dose management of immunosuppressant drugs (cyclosporine, tacrolimus, and mycophenolate) [80,82]. The side effects of these drugs can lead to treatment nonadherence, as shown by Almardini et al. [83], who reported 36% nonadherence to mycophenolate in a group of children. The economic cost and social implications of organ rejection due to treatment nonadherence among transplant recipients make it essential to search for a simpler and less invasive method of drug therapy monitoring [78].

Final considerations

Although the early detection of kidney disease through simple and accurate identification of biomarkers is essential, it has been explored by few studies. The studies in this review found DBS to be a promising alternative for quantifying the main biomarkers of kidney diseases, but sources of variability should be considered separately for each analyte. Practical applications should follow strict validation protocols that contain information about sample type, card type, volume, temperature, humidity, and hematocrit parameters. Moreover, the assessment should include control subjects to ensure quality. Finally, future research should include expressive samples of patients at different stages of kidney disease and report information on clinical parameters.

Notes

Conflicts of interest

No potential conflict of interest relevant to this article was reported.

Funding

None.

Author contributions

All the work was done by CN and VS.

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Table 2.

Studies involving measurement of endogenous biomarkers of kidney function through DBS samples

Author	Collection method	Sample size	Analytical technique	Storage and quality control	Sample (range or mean±SD)	Assessment of agreement/performance
Creatinine
Quraishi [56]	VB	60	Colorimetric assay	37 °C and 4 °C for 15–90 day	Creatinine range: 0.5–3.3 mg/dL	R=0.94, ICC=0.93
	Whatman				Serum creatinine: 1.99±0.64 mg/dL
					DBS creatinine: 1.92±0.55 mg/dL
Abraham [57]	VB	15	Enzymatic assay	4 °C for 7 day	DBS: 1.39±0.46 mg/dL	R=0.91, ICC=0.92
Abraham [57]	Whatman n3	15	Enzymatic assay	Matrix effect	Serum: 1.35±0.50 mg/dL	R=0.91, ICC=0.92
Silva [13]	VB, CB	106	Colorimetric (Jaffé) assay	Not reported	Adult: 57±12 yr	R=0.48
						Mean difference BA (LA): 0 (0.68 to –0.55)
						Diagnostic cutoff GFR <60 mL/min/1.73 m²
						CKD-EPI: DBS sensitivity 94%, DBS specificity 55%, precision 90%
Nakano [58]	VB	100	MS/MS	Not reported	Pediatric: 7.9 yr	Creatinine: 0.12–1.2 mg/dL
					Serum creatinine: 0.4 mg/dL	R=0.86
					Creatinine range: 0.12–1.2 mg/dL	Mean difference BA (LA): 0 (–0.087 to +0.09)
					Calibration curve: linearity (0.039–5.0 mg/dL)	Creatinine: 0.12–0.8 mg/dL
					Accuracy: 81.6%–104.9%	R=0.72 / DBS=0.565×creatinine
					CV: 0.1%–5.8%
						BA (LA): 0 (–0.081 to 0.091)
Bachini [59]	CB	9	FIA-MS	Not reported	Olympic athletes	CV=10.7%, ICC=0.57
	Whatman 903				Serum creatinine: 813.6±102.4 μmol/L (9.20±1.16 mg/dL)
					DBS creatinine: 812.4±108.1 μmol/L (9.19±1.22 mg/dL)
Dalton [43]	VB, CB	66	ID-LCMS	–80 °C	Adult: 24–88 yr	Sensitivity: 100%
	Whatman 903		Colorimetric enzymatic assay	Standard 914a	Venous DBS creatinine: 0.85±1.10 mg/dL	Specificity: 62.7%–94.9%
					Capillary DBS creatinine: 0.83±1.19 mg/dL
Sham [60]	VB	3	LC-MS/MS	2–8 °C	Creatinine: 2.5–20 μg/mL	Precision ≤6.3%, recovery 88%–94%, R²>0.99
Sham [60]	VB	3	PSI-MS/MS	2–8 °C	Creatinine: 2.5–20 μg/mL	Precision ≤6.3%, recovery 88%–94%, R²>0.99
Cystatin C
Vogl [40]	VB, CB	141	ELISA	–70 °C	ELISA	R=0.94
	Whatman 903		Nephelometry	Hematocrit^a)	Intra-assay CV: 5.4%, Inter-assay CV: 7.4%	Cystatin C: 0.51–1.02 mg/L
					Nephelometry	DBS sensitivity 94%, DBS specificity 55%
						Misclassified CKD stage: 31%
					Intra-assay CV: 4.2%, Inter-assay CV: 6.9%
						Correlation venous vs. capillary blood: R=0.97
Crimmins [61]	VB	82	ELISA	–70 °C	Adult: >50 yr	R=0.78
	Whatman 903				Mean cystatin C: 0.75 (0.41–1.39)	Regression: DBS=0.355+0.7×cystatin C
						Mean difference BA (LA): –0.2 (–0.45 to 0.1)
Crimmins [34]	VB	3,149	ELISA	≥32.2 °C, time before freezing (0–2, 3, 4–5, 6–7, and >8 day)	Adult: >50 yr	R²=0.78
Crimmins [34]	Whatman 903	3,149	ELISA	Volume^b)	Mean cystatin C: 1.2 (0.5–9.2)	Regression: DBS=0.43+0.84×cystatin C
Urea
Plumbe [62]	VB, CB	20	Enzymatic assay	Analysis: <7 day	CV: 6%	Venipuncture: R=0.99
				Hematocrit^a)		Regression: DBS=1.07×urea–0.6
						Capillary sample: R=0.99
						Regression: DBS=1.07×urea+0.1
Quraishi [63]	VB	75	Enzymatic assay	120 day (4 °C) or 90 day (37 °C)	Intra-assay CV=4.2%, Inter-assay CV=6.3%	R=0.97, ICC=0.96
Quraishi [63]	Whatman	75	Enzymatic assay	Hematocrit^c)	Intra-assay CV=4.2%, Inter-assay CV=6.3%	R=0.97, ICC=0.96

DBS, dried blood spots; SD, standard deviation; VB, venous blood; R, Pearson correlation coefficient; ICC, intraclass correlation coefficient; CB, capillary blood; BA (LA), Bland-Altman and limits of agreement; GFR, glomerular filtration rate; CKD-EPI, Chronic Kidney Disease Epidemiology Collaboration; MS/MS, tandem mass spectrometry; CV, coefficient of variation; FIA-MS, flow injection analysis-mass spectrometry; ID-LCMS, isotope dilution-liquid chromatography/mass spectrometry; LC, liquid chromatography; PSI, paper spray ionization; ELISA, enzyme-linked immunosorbent assay.

^a)Lowest influence or undefined variations in the assessed parameters. ^b)Presence or ^c)absence of statistical differences in biomarker concentrations according to variations in the assessed parameters.

Table 3.

Studies involving measurement of exogenous markers of kidney function through DBS samples

Author	Collection method	Sample size	Analytical technique	Storage and quality control	Study population	Assessment of agreement/performance
Iohexol
Niculescu-Duvaz [65]	VB, CB (3 points)	82	HPLC	–20 °C	Mean age: 41 yr	R²=0.953
	Schleicher & Schuell Grade 903			Hematocrit^a)
				Recovery^b)
Mafham [66]	VB, CB (3 points)	81	HPLC	Analysis: <4 hr	Mean age: 53±17 yr	Bias ±1.96×SD (mL/min/1.73 m²)
	Schleicher & Schuell Grade 903			Hematocrit^a)	GFR 15–124 mL/min/1.73 m²	3-spot iohexol clearance: 1.1±15.1
						2-spot iohexol clearance: 0.6±14.9
						1-spot iohexol clearance: 4.5±21.2
Maahs [67]	VB, CB (5 points)	15	HPLC	Analysis: <4 hr	Patients with type 1 diabetes	5-point blood spot GFR: 84.1±15.4 mL/min/1.73 m²(R=0.89), mean BA difference=0.16
	Whatman 903 Protein Saver			Hematocrit^a)	Mean age: 29±12 yr	2-point blood spot GFR: 83.4±15.4 mL/min/1.73 m² (R=0.89), mean BA difference=0.81
					Iohexol IV (1,500 mg)
Salvador [41]	VB, CB (7 points)	32	HPLC	Hematocrit^a)	Age: <6 yr	Median (range) reference GFR 65 (6–122) mL/min/1.73 m²; 2, 3, and 4-point blood spot GFR: R=0.947, R=0.945, and R=0.937, respectively
	Whatman 903 Protein Saver				Iohexol IV (647 mg/mL)	Diagnostic accuracy for 2-point blood spot: 87.5% and 96.9±15% (P15) and 96.9±30% (P30) of the reference GFR respectively
						GFR ˂60 mL/min/1.73 m², P15 and P30 accuracy 100%
Wang [68]	VB, CB (3 points)	45	Not reported	Not reported	Pediatric patients with chronic kidney disease	R=0.958
Wang [68]	VB, CB (3 points)	45	Not reported	Not reported	Pediatric patients with chronic kidney disease	Bias 4.26±9.06 mL/min/1.73 m²
Luis-Lima [69]	VB, CB (7 points)	203	HPLC	Volume^c)	Mean age: 57.3±15.3 yr	Capillary blood on card: total deviation index=26%
	Whatman 903				Mean GFR: 63.6±34.8 mL/min	Blood pipetted on card: total deviation index=13%
						In vivo studies: deviation index=9.5%
Staples [42]	VB, CB (4 points)	41	HPLC	Analysis: <5 hr	Age: 1–21 yr	Correlation between the DBS and 2-point venous GFR: R=0.95
	Schleicher & Schuell Grade 903			Hematocrit^d)	Iohexol IV (647 mg/mL)	2-point GFR±10% 4-point GFR: 94%
					Mean creatinine: 1.13±0.45 mg/dL	DBS GFR±10% 2-point GFR: 80%
Iothalamate
Hagan [53]	VB (6 points)	10	HPLC	Analysis: <5 hr	Mean age: 65.2±13.4 yr	Regression: slope of 0.95 (95% CI, 0.82–1.17)
	Whatman 903 Protein Saver			Hematocrit^c)	Mean GFR: 33.4±10.1 mL/min/1.73 m²	BA: bias (LA) 2 mL/min (–6 to 10 mL/min)
						Precision (% coefficient of variation): 3.2%–13.3%
						Accuracy (% error): 1.3%–3.7%

DBS, dried blood spots; VB, venous blood; CB, capillary blood; HPLC, high-performance liquid chromatography; SD, standard deviation; IV, intravenous; GFR, glomerular filtration rate; BA, Bland-Altman; R, Pearson correlation coefficient; CI, confidence interval; LA, limits of agreement.

^a)Concentration corrected according to a mathematical equation. ^b)Absence or ^c)presence of different statistics in marker oncentrations according to variations in the assessed parameters. ^d)Lowest influence or undefined variations in the assessed parameters.

Table 4.

Studies that simultaneously measured creatinine and medication clearance through DBS samples

Author	Assessed medication	Collection method	Sample size	Analytical technique	Storage and quality control	Study population (yr)	Calibration and performance
Scherf-Clavel [74,76]	Metformin and sitagliptin	VB, CB	70	LC-MS/MS, enzymatic assay	Volume^a)	Mean±SD: 67±11	Limit of quantification Cr: 0.15 mg/dL, Cf capillary vs. plasma=0.916±0.088
Scherf-Clavel [74,76]	Metformin and sitagliptin	VB, CB	70	LC-MS/MS, enzymatic assay	Volume^a)	Mean±SD: 67±11	R=0.944, mean BA deviation=0.001 mg/dL
Mathew [77]	Tacrolimus	VB, CB	131	LC-MS/MS	Time: 5 day	Range: 30–49	Imprecision <12% and limits of clinical acceptance within 15% against the venous samples
		Whatman 903			Temperature: ambient
					Hematocrit^b)
Koop [78]	Tacrolimus	VB, CB	21	LC-MS/MS	Time: 4 wk	Mean±SD: 14±4.6	Limit of quantification Cr 0.01 mg/dL, accuracy 7.94%
Koop [78]	Tacrolimus	FTA DMPK-A	21	LC-MS/MS	Temperature: ambient	Mean±SD: 14±4.6	Intra- and inter-day precision: 3.48%–4.11%
Al‐Uzri [39]	Tacrolimus	VB, CB	30 Subjects	LC-MS/MS, colorimetric assay, RIA	Time: 4 wk up to 1 mo on a dissected card	Mean±SD: 13.6±5.4	Correlation between DBS vs. intravenous samples: tacrolimus: R²=0.81
Al‐Uzri [39]	Tacrolimus	VB, CB	216 cards	LC-MS/MS, colorimetric assay, RIA	Temperature: ambient	Range: 2–21	Cr: R²=0.95
Francke [79]	Tacrolimus and cyclosporin	VB, CB	176	LC-MS/MS	Hematocrit^c)	Mean: 62	R=0.953
Veenhof [80]	Tacrolimus and cyclosporin	VB, CB	172 Subjects	LC-MS/MS, enzymatic creatinine assay	1–7 day at room temperature after: –20 °C	Mean±SD: 55±14	Correlation between DBS vs. intravenous samples
		Whatman DMPK-C	210 cards		Hematocrit^b)		Mean serum Cr: 149 µmol/L (n=199), R²=0.97, y=0.73x–1.55
							BA bias of −2.1 μmol/L (95% CI, −3.7 to −0.5)
							BA=[Cr serum µmol/L]=[DBS]/0.73
							Mean serum tacrolimus 7.1 μg/L (n=106), R²=0.93, y=1.0x–0.23, BA bias of −0.28 μg/L (95% CI, −0.45 to −0.12)
							Mean serum cyclosporine A 109 μg/L (n=61), R²=0.93, y=0.99x–1.86
Koster [32]	Tacrolimus, sirolimus, everolimus, and cyclosporin	VB,	50	LC-MS/MS, enzymatic assay	32 °C for 1 wk, –20 °C for 29 wk	Not available	Range for Cr: 7-point calibration curve (120–480 µmol/L), 1-point calibration curve (116–7,000 µmol/L), 8-point calibration curve (1–400 µmol/L)
		FTA DMPK-C			Volume^b)		Precision and accuracy (all validations): maximum CV of 14.0% and maximum bias of –5.9%
					Hematocrit^b)
Scribel [54]	Vancomycin	VB, CB	29 Subjects	LC-MS/MS	22 °C and 45 °C for 2 wk	Age: >18 yr	Cr validation: accuracy (99.6%–102.6%), intra-assay precision=2.6%–5.6%, inter-assay precision=3.5%–6.1%
		Whatman 903	54 Samples		Hematocrit^b)		DBS and serum comparison: accuracy (94.4%–102.6%), intra-assay precision=2.1%–5.6%, inter-assay precision=3.5%–7.0%
							Cr serum to DBS concentration ratio: 0.8–1.28; R=0.96
							Correlation between DBS vs. intravenous samples:
							Vancomycin: R²=0.89 (n=54) DBS capillary blood
							Vancomycin: R²=0.93 (n=19) DBS venous blood
							Cr: R²=0.95 (n=54)

DBS, dried blood spots; VB, venous blood; CB, capillary blood; LC, liquid chromatography; MS/MS, tandem mass spectrometry; SD, standard devation; Cr, creatinine; Cf, correction factor; R, Pearson correlation coefficient; BA, Bland-Altman; RIA, radioimmunoassay; CI, confidence interval; CV, coefficient of variation.

^a)Absence of differences in marker concentrations according to variations in the assessed parameters. ^b)Lowest influence on the assessed parameters. ^c)Concentration corrected according to a mathematical equation.